Lithium iron phosphate positive electrode material and preparation method thereof and lithium ion battery

Abstract

The present invention relates to the technical field of lithium ion battery materials, and in particular to lithium iron phosphate positive electrode materials, preparation methods thereof, and lithium ion batteries. The lithium iron phosphate positive electrode material crystals meet the following requirements: (1) #imgabs0#, 5nm 2 ≤S 2 (D)≤25nm 2 ; (2) 15nm≤D[200]≤85nm; (3) 30nm≤D a ≤100nm; where S 2 (D) represents the grain size variance of the characteristic growth crystal plane of the lithium iron phosphate positive electrode material crystal; D[200] represents the grain size of the [200] crystal plane of the lithium iron phosphate positive electrode material crystal; D a The average grain size of the lithium iron phosphate cathode material in all crystal directions is shown in Table 1. The lithium ion battery prepared from the lithium iron phosphate cathode material has good low-temperature electrochemical performance and high-rate charge and discharge performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion battery materials, and in particular to a lithium iron phosphate positive electrode material and a preparation method thereof, and a lithium ion battery. Background Art

[0002] In recent years, lithium-ion batteries have been widely used in portable electronic products and electric vehicles. Among them, lithium iron phosphate (LiFePO4) is the positive electrode material of lithium-ion batteries. It has the advantages of high theoretical specific capacity, good thermal stability, excellent safety performance, environmental friendliness and low price. However, the positive electrode material formed by lithium iron phosphate has the defects of rapid capacity decay during high-rate charge and discharge and poor low-temperature electrochemical performance, which greatly limits its further application in power batteries.

[0003] Specifically, the lower electronic conductivity and the lithium ion content in LiFePO 4 / The diffusion path between the FePO4 two-phase interface is long, resulting in a low diffusion rate. This results in the active material of the lithium iron phosphate cathode material not being fully utilized during high-current charge and discharge, increasing the electrode internal resistance and polarization, which leads to a rapid capacity decay during high-rate charge and discharge. At the same time, compared with other cathode materials, the reason why the low-temperature electrical performance of lithium iron phosphate cathode materials is not ideal is mainly related to the poor conductivity of the material and the increased obstruction of the charge transfer and solid-phase diffusion processes. The low temperature environment will hinder the diffusion of lithium ions, making the deintercalation / intercalation of lithium ions difficult, and the internal resistance and polarization of the battery will also increase. Therefore, the low-temperature performance of lithium batteries is poor. Moreover, under extreme overcharge or low temperature conditions, local lattice damage will be caused to the internal part of the lithium iron phosphate cathode material to a certain extent, introducing defects and affecting the crystal order, which will also affect the deintercalation / intercalation kinetics of lithium ions.

[0004] To address the above issues, a series of methods are currently being used to improve lithium iron phosphate positive electrode materials, such as carbon coating, ion doping, and particle nano-sizing. The above optimization methods have achieved certain results, but with the continuous expansion of the application fields of electric vehicles, the high-rate charge and discharge performance and low-temperature electrical performance of lithium iron phosphate batteries still need to be continuously improved.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The present invention aims to provide a lithium iron phosphate cathode material, a preparation method thereof, and a lithium ion battery. The present invention provides a lithium iron phosphate cathode material, which can form a lithium ion battery with good low-temperature electrochemical performance and high-rate charge and discharge performance.

[0007] The present invention is achieved in the following ways:

[0008] The present invention provides a lithium iron phosphate positive electrode material, wherein the lithium iron phosphate positive electrode material crystal meets the following requirements:

[0009] (1) , 5nm 2 ≤S 2 (D) ≤25nm 2 ; Among them, S 2 (D) represents the grain size variance of the characteristic growth crystal plane of the lithium iron phosphate positive electrode material crystal, in nm 2 ; N is 7, indicating the seven characteristic growth crystal planes of

[101] ,

[111] ,

[211] ,

[311] ,

[301] ,

[121] and

[040] of the lithium iron phosphate positive electrode material crystal; D i represents the grain size of the i-th characteristic growth crystal plane of the lithium iron phosphate positive electrode material crystal, in nm; represents the average value of the grain size of the seven characteristic growth crystal planes of the lithium iron phosphate positive electrode material crystal, in nm;

[0010] (2) 15 nm ≤ D

[200] ≤ 85 nm; D

[200] represents the grain size of the

[200] crystal plane of the lithium iron phosphate positive electrode material crystal, in nm;

[0011] (3) 30nm≤D a ≤100nm;D a It represents the average grain size of the lithium iron phosphate positive electrode material crystals in all crystal directions, in nm.

[0012] In an optional embodiment, 0.6≤ ≤0.9, It represents the ratio of the grain size of the

[200] crystal plane of the lithium iron phosphate positive electrode material crystal to the average grain size of all crystal directions of the lithium iron phosphate positive electrode material crystal.

[0013] In an alternative embodiment, , 0.03nm -2 ≤Q≤0.15nm -2 ; Wherein, Q represents the crystal parameter factor of the lithium iron phosphate positive electrode material, the unit is nm -2 .

[0014] In an optional embodiment, the lithium iron phosphate positive electrode material includes a lithium iron phosphate matrix and a carbon coating layer coated on the surface of the lithium iron phosphate matrix;

[0015] The general formula of the lithium iron phosphate matrix is ​​as follows: Li 1-x A x Fe 1-y M y (PO4-m )D m ; wherein A is selected from at least one of Na and Mg; M is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D is selected from at least one of F and S; 0≤x≤0.1; 0≤y≤0.1 and 0≤m≤0.1;

[0016] The content of the carbon coating layer coated on the surface of the lithium iron phosphate substrate is 1 wt % to 5 wt %.

[0017] In a second aspect, the present invention provides a method for preparing the lithium iron phosphate positive electrode material according to the aforementioned embodiment, comprising: heating a reaction solution containing phosphorus, iron and lithium elements using a plasma jet;

[0018] The precursor solution formed by the heat treatment and the polymer additive solution are stirred at high speed and ultrasonically treated, and then a hydrothermal reaction is carried out. After the reaction is completed, the solution is dried to obtain a lithium iron phosphate precursor;

[0019] The lithium iron phosphate precursor is mixed with a carbon source and then sintered to obtain a lithium iron phosphate positive electrode material.

[0020] In an optional embodiment, the plasma jet conditions include: an input voltage of 1000-1200 V, a treatment frequency of 15-20 kHz, an argon flow rate of 1.5-2 L / min, a distance between the plasma jet nozzle and the surface of the reaction solution of 5-8 cm; and a plasma jet treatment time of the reaction solution of 30-50 min.

[0021] The heating temperature is 70-80°C.

[0022] In an optional embodiment, the reaction solution further includes element M and / or element A;

[0023] The phosphorus element is derived from an organic phosphonic acid; the iron element is derived from a ferrous salt; the M element is derived from a water-soluble salt of the M element; the lithium element is derived from a lithium salt; and the A element is derived from a salt containing the A element.

[0024] The ratio of the total molar number of the iron element and the M element, the total molar number of the lithium element and the A element, and the molar number of the phosphorus element is 1:1:(0.95-1.1);

[0025] The molar ratio of the lithium element to the A element is (0.9-1): (0-0.1);

[0026] The molar ratio of the iron element to the M element is (0.9-1): (0-0.1);

[0027] The organic phosphonic acid is selected from any one of aminotrimethylphosphonic acid, hydroxyethyldiphosphonic acid and hydroxyphosphonoacetic acid.

[0028] In an optional embodiment, the polymer additive is selected from any one of hydroxypropyl methylcellulose, methylcellulose and polyvinyl pyrrolidone;

[0029] The carbon source is selected from carbohydrate compounds;

[0030] When mixed with a carbon source, it also includes adding a D source, wherein the D source is selected from a compound containing the D element;

[0031] The mass concentration of the polymer additive solution is 0.5wt%-1.0wt%;

[0032] The molar ratio of the iron element, the carbon source and the D source is 1: (0.05-0.15): (0-0.1).

[0033] In an optional embodiment, the high-speed stirring rate is 600-1000 r / min; the ultrasonic treatment time is 20-30 min;

[0034] The conditions of the hydrothermal reaction include: a temperature of 150-180°C and a time of 8-10 hours;

[0035] The sintering process includes: holding at 400-550°C for 2-5 hours, and then firing at 600-750°C for 6-8 hours.

[0036] In a third aspect, the present invention provides a lithium-ion battery comprising the lithium iron phosphate positive electrode material described in the aforementioned embodiment.

[0037] The present invention has the following beneficial effects: (1) The internal structure of the crystal of the lithium iron phosphate positive electrode material provided by the embodiment of the present invention is more ordered, the lithium ion diffusion path is more direct and effective, the diffusion resistance of lithium ions is reduced and the migration rate is increased, which is conducive to reducing the occurrence of polarization phenomenon in the charging and discharging process of the lithium iron phosphate battery.

[0038] (2) The lithium iron phosphate cathode material provided by the embodiment of the present invention has regularly shaped grains. The surface of the regularly shaped grains is more uniform and smooth, which is conducive to increasing the contact area between the particles and helping to form a continuous electron conduction network, thereby improving the conductivity of the lithium iron phosphate cathode material.

[0039] (3) The lithium iron phosphate positive electrode material provided by the embodiment of the present invention has fewer surface defects and stresses. Under extreme charge and discharge conditions, it can suppress the generation of local lattice defects in the lithium iron phosphate positive electrode material, reduce the stress generated during the lithium ion extraction / insertion process, and thus reduce the internal resistance to lithium ion migration.

[0040] (4) In the lithium iron phosphate positive electrode material provided by the embodiment of the present invention, the diffusion distance of lithium ions in the crystal of the positive electrode material is short, the migration rate during the insertion and removal process is fast, and the polarization generated by the electrode is small. As a result, the lithium ion battery can be charged and discharged under high rate conditions, and the lithium battery has low temperature adaptability.

[0041] (5) The preparation method provided in the embodiment of the present invention is conducive to promoting the rapid nucleation of lithium iron phosphate crystals and adjusting the structure and morphology of the crystals, so that the grain size of the lithium iron phosphate crystals is isotropic, which is conducive to the formation of regular crystals and orderly arrangement, can eliminate the internal stress of the lithium iron phosphate positive electrode material and improve the crystallinity and conductivity of the material, thereby ensuring the performance of the lithium iron phosphate positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0043] Figure 1 The XRD patterns of the lithium iron phosphate cathode materials prepared in Example 1 and Comparative Example 2 are shown;

[0044] Figure 2 This is a SEM image of the lithium iron phosphate cathode material prepared in Example 1;

[0045] Figure 3 This is the SEM image of the lithium iron phosphate positive electrode material prepared in Comparative Example 4. DETAILED DESCRIPTION

[0046] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be purchased commercially.

[0047] An embodiment of the present invention provides a lithium iron phosphate positive electrode material. A battery containing the lithium iron phosphate positive electrode material has good low-temperature electrochemical performance and high-rate charge and discharge performance.

[0048] Specifically, the crystals of the lithium iron phosphate positive electrode material meet the following requirements:

[0049] (1) 5nm 2 ≤S 2 (D) ≤25nm 2; For example, 5 nm 2 , 10 nm 2 , 15 nm 2 , 20 nm 2 , 25 nm 2 or 5-25nm 2 Any value between . Among them, S 2 (D) represents the grain size variance of the characteristic growth crystal plane of the lithium iron phosphate positive electrode material crystal, and its unit is nm 2 ;S 2 (D) reflects the dispersion of grain sizes corresponding to different growth planes in the lithium iron phosphate cathode material relative to the average value, and is used to characterize the degree to which the grain shape in the lithium iron phosphate cathode material tends to be regular. Generally, regularly shaped grains indicate a more ordered internal structure, which helps form a more direct and efficient lithium ion diffusion path, reduces lithium ion diffusion resistance, increases its migration rate, and reduces polarization during charge and discharge. Furthermore, regularly shaped grains have a more uniform and smoother surface, which helps increase the contact area between particles and helps form a continuous electron conduction network, thereby improving the conductivity of the lithium iron phosphate cathode material.

[0050] Specifically, S 2 The smaller the (D) value, the more regular the grain shape, indicating that the growth rate of the crystal in the lithium iron phosphate cathode material in each growth crystal plane direction is more uniform, its grain size is basically isotropic, and the internal structure of the material crystal tends to be ordered; while S 2 The larger the (D) value, the uneven growth rate of the crystals in the lithium iron phosphate cathode material in all directions, showing anisotropy, the resulting grain shape is irregular, and the internal structure of the material crystal tends to be disordered, which is not conducive to the diffusion and transfer of lithium ions. Therefore, generally S 2 (D) The smaller the better, but S 2 (D) is too small to be easily realized, that is, in the embodiment of the present invention, S 2 (D) less than 5 nm 2 It is not easy to achieve, and S 2 (D) greater than 25 nm 2 , it is impossible to effectively improve the low-temperature electrochemical performance and high-rate charge and discharge performance of the lithium-ion battery formed by the lithium iron phosphate positive electrode material.

[0051] Furthermore, S 2 (D) is calculated as follows: , where N is 7, representing the following 7 characteristic growth crystal planes of lithium iron phosphate positive electrode material crystals, namely

[101] ,

[111] ,

[211] ,

[311] ,

[301] ,

[121] and

[040] crystal planes. irepresents the grain size of the i-th characteristic growth crystal plane of the lithium iron phosphate positive electrode material crystal, in nm; represents the average value of the grain size of the seven characteristic growth crystal planes of the lithium iron phosphate positive electrode material crystal, in nm;

[0052] It should be noted that: (1) The corresponding relationship between the main diffraction peak positions of lithium iron phosphate XRD and the crystal plane is: 20.85°

[101] , 25.63°

[111] , 29.61°

[211] , 35.67°

[311] , 32.26°

[301] , 36.53°

[121] and 61.77°

[040] .

[0053] (2) 15nm≤D

[200] ≤85nm; for example, 15 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 85 nm, or any value between 15 and 85 nm. D

[200] represents the grain size of the

[200] crystal plane of the lithium iron phosphate positive electrode material crystal, and its unit is nm. The

[200] crystal plane has a low surface energy and is an advantageous crystal plane for reducing surface defects and stress in the lithium iron phosphate positive electrode material. Under extreme charge and discharge conditions, the presence of this crystal plane not only helps to suppress local lattice defects that may be generated in the lithium iron phosphate positive electrode material, but also helps to reduce the stress generated during the lithium ion deintercalation / embedding process, and to a certain extent, can reduce the internal resistance to lithium ion migration. The embodiment of the present invention adopts D

[200] in the above range to improve the performance of the lithium iron phosphate positive electrode material. It is difficult to achieve D

[200] below 15nm in the embodiment of the present invention, and if D

[200] is greater than 85nm, the performance of the lithium iron phosphate positive electrode material formed will be reduced, which will in turn lead to poor low-temperature electrochemical performance or high-rate charge and discharge performance of the lithium-ion battery formed therefrom.

[0054] (3) 30nm≤D a ≤100 nm; for example, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any value between 30 and 100 nm. a It represents the average grain size of all crystal directions of the lithium iron phosphate positive electrode material crystal, and its unit is nm. aIt can reflect the macroscopic size characteristics of the material crystal. Generally, the smaller the average grain size, the shorter the diffusion distance of lithium ions in the crystal of lithium iron phosphate cathode material, the faster the migration rate during the deintercalation process, and the smaller the polarization generated by the electrode, which is conducive to the formation of lithium-ion batteries for charging and discharging under high rate conditions and its low temperature adaptability; and small grain size is often accompanied by a large specific surface area, and the large surface is conducive to the subsequent uniform distribution of the carbon coating layer on the surface of the lithium iron phosphate matrix material particles, further improving the conductivity of the lithium iron phosphate cathode material. And D a If the thickness is less than 30 nm, it is difficult to achieve, and if it is greater than 100 nm, the low-temperature electrochemical performance or high-rate charge and discharge performance of the formed lithium-ion battery will be poor.

[0055] When the lithium iron phosphate positive electrode material provided by the embodiment of the present invention satisfies the above three requirements at the same time, the lithium ion battery formed therefrom has good low-temperature electrochemical performance and high-rate charge and discharge performance.

[0056] It should be noted that the grain sizes of the different characteristic growth crystal planes and the

[200] crystal plane of the above-mentioned lithium iron phosphate positive electrode material crystals are obtained by MDI Jade performing full spectrum fitting on the X-ray diffraction curve of the lithium iron phosphate positive electrode material and calculating using the Scherrer formula; the average grain size of all crystal directions of the lithium iron phosphate positive electrode material crystals is obtained by MDI Jade performing full spectrum fitting on the X-ray diffraction curve of the lithium iron phosphate positive electrode material and calculating using the Cauchy distribution.

[0057] The lithium iron phosphate cathode material crystal provided by the embodiment of the present invention not only meets the requirements of S 2 (D), D

[200] and D a If the requirement of 0.6≤ The requirement of ≤0.9, for example, 0.6, 0.7, 0.8, 0.9 or any value between 0.6 and 0.9, and the electrochemical performance of the obtained lithium iron phosphate positive electrode material under extreme conditions is improved.

[0058] Specifically, The ratio of the grain size of the

[200] crystal plane of the lithium iron phosphate positive electrode material crystal to the average grain size of all crystal directions of the lithium iron phosphate positive electrode material crystal is 0.6≤ ≤0.9. It can reflect the exposure degree of

[200] crystal plane in the lithium iron phosphate cathode material crystal; usually, D

[200] / D a The smaller the value, the less the low surface energy

[200] crystal plane is exposed in the lithium iron phosphate cathode material crystal, the more stress is generated during the lithium ion deintercalation / embedding process, and the conduction of lithium ions is hindered; D

[200] / D aThe larger the value, the more low surface energy

[200] crystal plane is exposed, the less surface lattice defects are generated in the lithium iron phosphate cathode material during charge and discharge under extreme conditions, and the electrochemical stability is improved. However, since the values ​​of D

[200] and Da must be controlled within an appropriate and balanced range, a too high D

[200] / D a The value is difficult to achieve, too low D

[200] / D a This will lead to a decrease in the performance of the lithium iron phosphate positive electrode material.

[0059] Furthermore, the lithium iron phosphate cathode material crystal provided by the embodiment of the present invention not only satisfies S 2 (D), D

[200] , D a and If the requirement of 0.03nm is further met -2 ≤ ≤0.15nm -2 , for example 0.03 nm 2 , 0.04nm 2 , 0.05 nm 2 , 0.06 nm 2 , 0.07 nm 2 , 0.08 nm 2 , 0.09 nm 2 , 0.1 0nm 2 , 0.11 nm 2 , 0.12 nm 2 , 0.13nm 2 , 0.14 nm 2 , 0.15 nm 2 or 0.03-0.15 nm 2 The obtained lithium iron phosphate positive electrode material has better high-rate electrical performance and low-temperature electrical performance.

[0060] Specifically, the crystal parameter factor of the lithium iron phosphate cathode material obtained from the above relationship is recorded as Q, and its unit is nm -2 This index is consistent with the grain size variance S of the characteristic growth crystal plane of the lithium iron phosphate cathode material crystal. 2 (D), the grain size D

[200] of the

[200] crystal plane and the average grain size D of all crystal directions of the lithium iron phosphate positive electrode material crystal a The corresponding relationship can comprehensively reflect the electrochemical performance of lithium iron phosphate cathode materials. 2 (D), D aThe smaller the value, the larger the D

[200] / Da value, and the larger the crystal parameter factor Q value of the lithium iron phosphate cathode material, that is, the smoother the lithium ion transmission channel of the lithium iron phosphate cathode material during charging and discharging under extreme conditions, the higher the electronic conductivity of the lithium iron phosphate cathode material, and the corresponding reduction of the battery internal resistance and polarization phenomenon; furthermore, the electrochemical performance of the lithium iron phosphate battery will also be improved. In theory, the larger the crystal parameter factor Q value, the better, but it is technically difficult to achieve a Q value that is too large, and a Q value that is too small will lead to a decrease in the performance of the lithium iron phosphate cathode material.

[0061] In summary, lithium iron phosphate cathode materials meet the requirements of S 2 (D), D

[200] , D a 、 When the conditions of Q and Q are met, the performance of the lithium iron phosphate positive electrode material is optimal, and the lithium-ion battery prepared from the positive electrode material has better low-temperature electrochemical performance and high-rate charge and discharge performance.

[0062] Furthermore, the lithium iron phosphate positive electrode material includes a lithium iron phosphate matrix and a carbon coating layer coated on the surface of the lithium iron phosphate matrix. The general formula of the lithium iron phosphate matrix is ​​as follows: Li 1-x A x Fe 1-y M y (PO 4-m )D m A is selected from at least one of Na and Mg; M is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, and Y; and D is selected from at least one of F and S. 0 ≤ x ≤ 0.1; 0 ≤ y ≤ 0.1, and 0 ≤ m ≤ 0.1. The carbon coating layer coated on the surface of the lithium iron phosphate substrate has a content of 1 wt% to 5 wt%, for example, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, or any value between 1 and 5 wt%.

[0063] In a second aspect, the present invention provides a method for preparing the lithium iron phosphate positive electrode material described in the aforementioned embodiment, comprising:

[0064] S1, forming a reaction solution;

[0065] Raw materials containing iron and lithium elements are dispersed in deionized water respectively, for example, ferrous salt containing iron and lithium source containing lithium are dispersed in deionized water, and an inert gas (for example, including but not limited to nitrogen) is blown under stirring conditions to remove dissolved oxygen in the solution to obtain a metal salt solution.

[0066] The metal salt solution may also include element M and / or element A. For example, ferrous salt and a lithium source, an M source containing element M and / or an A source containing element A are dispersed in the above-mentioned deionized water, and an inert gas is blown under stirring conditions to obtain a metal salt solution.

[0067] A raw material containing phosphorus element, namely a phosphorus source, is dispersed in an alcohol solvent (for example, including but not limited to ethanol) to obtain a phosphorus source solution.

[0068] The metal salt solution and phosphorus source solution are mixed and stirred under a protective gas atmosphere (e.g., including but not limited to nitrogen) to obtain a mixed solution, and the pH of the mixed solution is then adjusted to 6-8 to form a reaction solution. As can be seen, the reaction solution contains phosphorus, iron, and lithium, and may also contain element M and / or element A.

[0069] Wherein, phosphorus element is derived from organic phosphonic acid, for example, including but not limited to any one of aminotrimethylphosphonic acid, hydroxyethyldiphosphonic acid and hydroxyphosphonoacetic acid. Iron element is derived from ferrous salt, for example, including but not limited to any one of ferrous oxalate, ferrous chloride and ferrous acetate. M element is derived from the water-soluble salt of M element, specifically the water-soluble salt of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y element. A element is derived from the salt containing A element, for example, including but not limited to any one of sodium fluoride, magnesium fluoride, sodium chloride, magnesium chloride and sodium sulfide. Lithium element is derived from lithium salt, for example, including but not limited to any one of lithium oxalate, lithium chloride and lithium acetate.

[0070] Furthermore, the raw materials containing the above elements are added in amounts according to the chemical formula (molar ratio). For example, the ratio of the total molar number of the iron element to the M element, the total molar number of the lithium element to the A element, and the molar number of the phosphorus element is 1:1:(0.95-1.1); the molar ratio of the lithium element to the A element is (0.9-1):(0-0.1); and the molar ratio of the iron element to the M element is (0.9-1):(0-0.1).

[0071] S2, heating treatment;

[0072] The reaction solution is heated by using a plasma jet.

[0073] Plasma jet is a mature plasma generation technology. When high-energy particles within the discharge filaments of the device bombard a mixed solution of metal salts and organic phosphonic acid, they impart high energy to the mixture, which promotes the rapid nucleation of crystals of the lithium iron phosphate cathode material and forms primary crystal nuclei, thereby shortening the crystallization time during the hydrothermal reaction. Furthermore, the plasma generates a large number of highly active radicals, which trigger a self-regulating effect in the reaction system and reduce the supersaturation of the solution, which is beneficial for adjusting the crystal structure and morphology of the lithium iron phosphate cathode material. In the subsequent hydrothermal reaction process, the organic phosphonic acid can not only serve as a phosphorus source for synthesizing lithium iron phosphate, but also act as a chelating agent (coordination effect of ferrous ions and hydroxyl, amino or carboxyl groups) to be adsorbed onto the primary particles of the lithium iron phosphate positive electrode material to adjust the surface structure of the crystal, further reducing the surface energy of the

[200] crystal plane. The reduction in surface energy will slow down the growth rate of the

[200] crystal plane and tend to stabilize it, and it will not be consumed rapidly, thereby allowing the crystals of the lithium iron phosphate positive electrode material to obtain more

[200] crystal plane orientations; in addition, low supersaturation will slow down the growth rate of the crystal, resulting in smaller differences in the growth rates of different crystal planes, which is conducive to obtaining crystals of lithium iron phosphate positive electrode material with isotropic grain size.

[0074] The plasma technology device for implementing the aforementioned plasma jet is an existing device. The present invention illustrates the following process using the plasma technology device: the reaction solution is poured into a conical flask and placed in a water bath, continuously stirred. A condenser is attached to the conical flask to prevent solvent evaporation. The plasma experimental apparatus consists of a plasma generator, a plasma nozzle, and an argon gas supply system. During the reaction, the plasma generator applies a high-voltage electric field, ionizing the atoms in the incoming gas to produce positive ions and free electrons, thereby forming a low-temperature plasma. An atmospheric pressure spray gun then utilizes the interaction between the electric field and the gas flow to eject the low-temperature plasma from the nozzle onto the surface of the reaction solution in the conical flask for reaction.

[0075] Therefore, the conditions of the plasma jet in the embodiment of the present invention include: an input voltage of 1000-1200 V, for example, 1000 V, 1100 V, 1200 V, or any value between 1000-1200 V. A processing frequency of 15-20 kHz, for example, 15 kHz, 16 kHz, 17 kHz, 18 kHz, 19 kHz, 20 kHz, or any value between 15-20 kHz. An argon flow rate of 1.5-2 L / min, for example, 1.5 L / min, 1.6 L / min, 1.7 L / min, 1.8 L / min, 1.9 L / min, 2.0 L / min, or any value between 1.5-2.0 L / min. A distance between the plasma jet nozzle and the surface of the reaction solution is 5-8 cm, for example, 5 cm, 6 cm, 7 cm, 8 cm, or any value between 5-8 cm. The plasma jet treatment time of the reaction solution is 30-50 min; for example, 30 min, 35 min, 40 min, 45 min, 50 min or any value between 30-50 min.

[0076] The aforementioned conditions facilitate heat treatment, enabling the lithium iron phosphate cathode material to have an isotropic grain size. Using other heat treatment methods, such as conventional water bath heating, to treat the reaction solution fails to achieve isotropic grain size, and the resulting battery subsequently fails to meet low-temperature electrochemical performance and high-rate charge-discharge performance requirements.

[0077] The temperature of the above-mentioned heating treatment is 70-80℃, for example, 70℃, 71℃, 72℃, 73℃, 74℃, 75℃, 76℃, 77℃, 78℃, 79℃, 80℃ or any value between 70-80℃.

[0078] S3, forming a lithium iron phosphate precursor;

[0079] The precursor solution formed by the above heating treatment and the polymer additive solution are stirred at high speed and ultrasonically treated.

[0080] Specifically, in the embodiment of the present invention, the polymer additive is selected from any one of hydroxypropyl methylcellulose, methylcellulose and polyvinyl pyrrolidone. The use of the above polymer additive is more conducive to achieving its role as a bridging agent and delaying solute diffusion.

[0081] The polymer additive is added in the form of a solution, and the mass concentration of the polymer additive solution is 0.5 wt%-1.0 wt%; for example, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt% or any value between 0.5-1.0 wt%.

[0082] It should be noted that the solvent for dissolving the polymer additive is any solvent known to dissolve the polymer additive, and may be water or another solvent. If both water and another solvent are suitable, water is preferred. The viscosity of the polymer additive increases with its mass concentration. Using the above mass concentration of the polymer additive can slow solute diffusion, accelerate localized crystal nucleation, and produce a large number of fine crystals.

[0083] The amount of the polymer additive solution can be adjusted according to production requirements. For example, in the embodiment of the present invention, 5-10 ml of the polymer additive solution is added to every 500 ml of the precursor solution.

[0084] Furthermore, the embodiments of the present invention use high-speed stirring and ultrasonic treatment to reduce the crystal size of the primary crystal nuclei to promote the subsequent formation of small particles. Specifically, the high-speed stirring rate is 600-1000 r / min; for example, 600 r / min, 700 r / min, 800 r / min, 900 r / min, 1000 r / min, or any value between 600-1000 r / min. The ultrasonic treatment time is 20-30 min; for example, 20 min, 21 min, 22 min, 23 min, 24 min, 25 min, 26 min, 27 min, 28 min, 29 min, 30 min, or any value between 20-30 min.

[0085] After the ultrasonication is completed, the mixed solution is transferred to a stainless steel high-pressure reactor lined with polytetrafluoroethylene and placed in an oven at 150°C-180°C for 8-10 hours. After the hydrothermal reaction is completed, the product is cooled to room temperature and washed, for example, by centrifugation with deionized water and anhydrous ethanol, and finally dried to obtain a lithium iron phosphate precursor; for example, dried in a vacuum oven at 70°C for 12 hours.

[0086] During the hydrothermal reaction, the polymer additive with a certain viscosity used in the embodiment of the present invention can delay the diffusion of the solute, thereby accelerating the local nucleation of the crystals and precipitating a large number of fine crystals. At the same time, the polymer additive can act as a bridging agent to agglomerate the fine crystals into regular crystals and arrange them in an orderly manner, ultimately obtaining lithium iron phosphate precursor particles with good crystallinity.

[0087] S4, forming a lithium iron phosphate positive electrode material;

[0088] Next, the lithium iron phosphate precursor is mixed with a carbon source and ball-milled. If the lithium iron phosphate cathode material contains element D, the lithium iron phosphate precursor is then mixed with a carbon source and a D source containing element D and ball-milled, wherein the molar ratio of the iron element, the carbon source, and the D source is 1:(0.05-0.15):(0-0.1).

[0089] It should be noted that the D source can be a sulfide or a fluoride, including but not limited to sodium sulfide, potassium sulfide, sodium fluoride, potassium fluoride and ammonium fluoride.

[0090] Specifically, the sintering process includes: heat preservation at 400-550°C for 2-5 hours, and then calcination at 600-750°C for 6-8 hours.

[0091] In the embodiment of the present invention, lithium iron phosphate precursor particles are ball-milled and mixed with a carbon source, and then placed in a muffle furnace for low-temperature and high-temperature calcination to obtain a lithium iron phosphate positive electrode material. The internal stress of the lithium iron phosphate positive electrode material is then eliminated, which can improve the crystallinity and conductivity of the lithium iron phosphate positive electrode material.

[0092] In a third aspect, the present invention provides a lithium-ion battery comprising the lithium iron phosphate positive electrode material described in the aforementioned embodiment.

[0093] The embodiment of the present invention uses the following detection method to test the relevant performance:

[0094] 1. The phase structure of the lithium iron phosphate cathode material was tested using an X-ray diffractometer (XRD). The acquired XRD data were then fitted with the full spectrum using MDI Jade until the fitting error factor R ≤ 15%. After fitting, the data were exported and the average grain size of the lithium iron phosphate cathode material and the grain size of the crystal plane corresponding to a certain diffraction angle were calculated using the Cauchy distribution and Scherrer formula.

[0095] (1) Calculation of the average grain size D of lithium iron phosphate cathode material using the Jade-Cauchy step-by-step method a

[0096] Cauchy distribution:

[0097] (n and k are set to 1)

[0098]

[0099] in, is the diffraction angle (2 ) half; is the half-height width under the diffraction peak corresponding to a certain diffraction angle, in radians; is the wavelength of incident X-rays, usually CuKα is selected. =0.15406nm; Da is the average grain size, in nm, Represents microscopic strain.

[0100] The original XRD data of the test sample were fitted by Jade (diffraction angle: 10°~90°) and substituted into the above formula for processing. A scatter plot is made with (FWHM×cosθ) / λ as the horizontal coordinate and (FWHM×cosθ) / λ as the vertical coordinate. The scatter plot is then linearly fitted to obtain a fitting straight line and a fitting equation (a linear equation of two variables). The average grain size of the material is calculated based on the intercept 1 / Da value of the fitting equation.

[0101] (2) The Scherrer formula is used to calculate the grain size D of the crystal plane corresponding to a certain diffraction angle of the lithium iron phosphate positive electrode material.

[0102]

[0103] Where k is a constant, generally taken as 0.89.

[0104] The original XRD data of the test sample were processed by Jade fitting and then input into the above formula to calculate the grain size of the corresponding crystal plane (

[101] ,

[111] ,

[211] ,

[311] ,

[301] ,

[121] ,

[040] and

[200] crystal planes) at a certain diffraction angle.

[0105] 2. Specific surface area test

[0106] The specific surface area of ​​the powder particles was measured using a Micromeritics ASAP2460 fully automatic specific surface analyzer. The nitrogen adsorption and desorption curve data at the medium and low pressure stages were analyzed and calculated using the BET formula to obtain the specific surface area of ​​the material in m 2 / g.

[0107] 3. Compaction density test

[0108] The compaction density of the powder particles was measured using a powder compaction density meter with the pressure set to 3T in units of g / cm 3 .

[0109] 4. Low temperature charge transfer impedance test

[0110] A low-temperature charge transfer impedance test was performed on a lithium-ion battery using lithium iron phosphate as the positive electrode using an electrochemical workstation with a temperature control system and EIS support. Before the test, the sample was pretreated to ensure that the electrode surface was clean and the electrolyte in the battery chamber was sufficient. The sample was then placed in a low-temperature environment and the temperature was set to -30°C. After a period of time to allow the sample temperature to stabilize uniformly, the impedance test was started. During the test, the open circuit voltage of the system was recorded, and the scanning frequency range was set to 0.01Hz~100kHz. The amplitude used the default value of 0.005V. The impedance data was recorded during the process. After the test, the test data was analyzed and fitted using software tools to extract the parameters related to the charge transfer process, namely the low-temperature charge transfer resistance R. ct .

[0111] 4. Electrode resistivity

[0112] The resistivity (Ω·cm) of the lithium iron phosphate positive electrode sheet was tested using a two-probe test method at room temperature (25°C), and the current intensity was set to 2A.

[0113] 5. Electrochemical performance test

[0114] Electrode preparation and button cell assembly: The lithium iron phosphate positive electrode material, the conductive agent acetylene black and the adhesive polyvinylidene fluoride were uniformly mixed in N-methylpyrrolidone at a mass ratio of 92:4:4 to make a slurry, which was then coated on aluminum foil and dried in a vacuum drying oven. The slurry was then pressed into a positive electrode sheet using a tablet press. The negative electrode sheet was a metallic lithium sheet. The electrolyte was 1 mol / L lithium hexafluorophosphate-ethylene carbonate: dimethyl carbonate (LiPF6-EC:DMC, volume ratio of 1:1). A polypropylene porous membrane was used as a separator. The battery was assembled in an argon glove box.

[0115] Battery rate test steps: Divide the assembled button batteries into a normal temperature group and a low temperature group; the voltage test platform consists of a high and low temperature test chamber and a battery charge and discharge tester. During the test, the temperature of the test chamber is adjusted to 25°C (normal temperature group) or -30°C (low temperature group). The charge and discharge voltage range is 2.5-4.5V. First, charge to 4.5V with a constant current, then discharge to 2.5V with a rate current. The released capacity is the discharge capacity at that rate. After the discharge is completed, charge to 4.5V with a constant current; then proceed to the next rate test.

[0116] The features and performance of the present invention are further described in detail below with reference to the embodiments.

[0117] Example 1-Example 13

[0118] Examples 1 to 13 and Comparative Examples 1 to 4 respectively provide a lithium iron phosphate positive electrode material, wherein the S of the lithium iron phosphate positive electrode material is 2(D), D

[200] , D a 、 , Q, the content of the carbon coating layer, and the chemical formula of the lithium iron phosphate matrix are all shown in Tables 1 and 2.

[0119] Table 1 Crystal parameters of lithium iron phosphate positive electrode materials of Examples 1-8 and Comparative Example 1

[0120]

[0121] Table 2 Crystal parameters of lithium iron phosphate positive electrode materials of Examples 9-13 and Comparative Examples 2-4

[0122]

[0123] Examples 1 to 13 and Comparative Examples 1 to 4 respectively provide methods for preparing lithium iron phosphate positive electrode materials. The preparation method of the lithium iron phosphate positive electrode material of Example 1 is taken as an example for description.

[0124] The preparation method of the lithium iron phosphate positive electrode material of Example 1 is as follows:

[0125] S1. Weigh ferrous chloride and lithium oxalate and disperse them in deionized water. Blow nitrogen gas for 1 h under stirring to remove dissolved oxygen in the solution to obtain a metal salt solution.

[0126] Aminotrimethylphosphonic acid was weighed and dispersed in an ethanol solution to obtain a phosphorus source solution.

[0127] The metal salt solution and phosphorus source solution were stirred under a nitrogen atmosphere for 30 minutes to obtain a mixed solution. Subsequently, the resulting mixed solution was adjusted to a pH of 7 with aqueous ammonia to form a reaction solution, which was then heated in a water bath in a plasma jet apparatus with continuous stirring during the reaction. Upon completion of the reaction, a precursor solution was obtained.

[0128] The plasma technology device treatment process involves pouring the reaction solution into a conical flask and placing it in a water bath with continuous stirring. A condenser is connected to the conical flask. The plasma experimental device consists of a plasma generator, a plasma nozzle, and an argon gas supply system. The main technical parameters of the experimental device during the reaction are: input voltage of 1100V, treatment frequency of 15kHz, argon flow rate of 1.5L / min, distance between the nozzle and the reaction solution surface of 6cm, plasma jet treatment time of 35 minutes, and water bath heating temperature of 75°C. The molar ratio of iron, lithium, and phosphorus is 1:1:1.

[0129] S2. Mix 7.5 mL of 0.6 wt % hydroxypropyl methylcellulose solution with 500 mL of the precursor solution obtained in step S1 under high-speed stirring at a stirring rate of 750 r / min under nitrogen atmosphere, and perform ultrasonic treatment for 25 min.

[0130] The precursor mixture was then transferred to a Teflon-lined stainless steel autoclave and placed in a 160°C oven for 9 hours. After the reaction, the product was cooled to room temperature and washed by centrifugation with deionized water and then anhydrous ethanol. Finally, it was dried in a vacuum oven at 70°C for 12 hours to obtain a lithium iron phosphate precursor.

[0131] The lithium iron phosphate precursor and glucose were ball-milled for 1 hour to obtain a mixed precursor. The fully ground precursor was then sintered in a muffle furnace filled with high-purity argon. The temperature was raised to 450°C at a rate of 5°C / min and held for 4 hours. The temperature was then raised to 650°C and calcined for 6 hours to obtain the lithium iron phosphate cathode material. The molar ratio of iron to glucose in the lithium iron phosphate was 1:0.10.

[0132] The preparation methods of the lithium iron phosphate positive electrode materials of Examples 2-8 and Comparative Example 1 refer to the preparation method of Example 1, with the only difference being some conditions. For specific conditions, see Tables 3 and 4.

[0133] Table 3 Specific conditions for the preparation method of lithium iron phosphate positive electrode material (1)

[0134]

[0135] Table 4 Specific conditions for the preparation method of lithium iron phosphate positive electrode material (2)

[0136]

[0137] The preparation methods of the lithium iron phosphate positive electrode materials provided in Examples 9 to 13 and Comparative Examples 2 to 4 also refer to the preparation method provided in Example 1, with only some operations being different, as follows:

[0138] Example 9: Compared with Example 1, the time for treating the mixed liquid with the plasma device in step S1 is changed to 45 min, the mass concentration of the polymer additive in S2 is 0.8 wt %, and the amount is 8.0 mL. The other steps remain unchanged.

[0139] Example 10: Compared with Example 1, the high-speed stirring rate in step S2 is changed to 900 r / min, the ultrasonic treatment time is 30 min, and the other steps remain unchanged.

[0140] Example 11: Compared with Example 1, the organic phosphine source in step S1 is changed to hydroxyphosphonoacetic acid, and the other steps remain unchanged.

[0141] Example 12: Compared with Example 1, the hydrothermal reaction temperature in step S2 is changed to 180°C and the reaction time is changed to 8 hours, and the other steps remain unchanged.

[0142] Example 13: Compared with Example 1, the low-temperature roasting temperature in step S2 is changed to 500°C and the time is changed to 5 hours; the high-temperature roasting temperature is changed to 700°C and the time is changed to 7 hours, and the other steps remain unchanged.

[0143] Comparative Example 2: Compared with Example 1, the phosphorus source in step S1 was changed to ammonium dihydrogen phosphate, the stirring rate in step S2 was changed to 200 r / min, ultrasonic treatment was not performed, and the other steps remained unchanged.

[0144] Comparative Example 3: Compared with Example 1, no plasma device is used for treatment during water bath heating in step S1, no polymer additive is added in step S2, and the remaining steps remain unchanged.

[0145] Comparative Example 4: Compared with Example 1, the phosphorus source in step S1 was changed to ammonium dihydrogen phosphate, and the plasma device was not used for treatment during the water bath heating process; the stirring rate in step S2 was changed to 200 r / min, ultrasonic treatment was not performed, and polymer additives were not added. The remaining steps remained unchanged.

[0146] Test Example 1

[0147] The performance tests were performed on the lithium iron phosphate cathode materials of Examples 1 to 13 and Comparative Examples 1 to 4. The results are shown in Tables 5 and 6.

[0148] Table 5 Test results of lithium iron phosphate positive electrode materials of Examples 1-8 and Comparative Example 1

[0149]

[0150] According to Table 1 and Table 5, the lithium iron phosphate cathode materials have the same chemical composition, corresponding to different S 2 (D), Da, D

[200] and D

[200] / D a , thus obtaining different crystal parameter factors Q, and then the lithium iron phosphate cathode material exhibits different electrochemical properties. The details are as follows:

[0151] (1) From Examples 1-8, it can be seen that S 2The smaller the (D) and Da values, the larger the D

[200] / Da value, and the larger the crystal parameter factor Q value obtained, the lithium iron phosphate positive electrode material exhibits better low-temperature electrochemical performance and high-rate charge-discharge performance. The reason is that the present invention adjusts the above parameters to make the lithium iron phosphate positive electrode material crystal obtain a more regular grain shape and a smaller average grain size, and increases the exposure degree of the low surface energy

[200] crystal plane in the material crystal. These factors are not only conducive to improving the specific surface area and compaction density of the lithium iron phosphate positive electrode material, but also conducive to the lithium iron phosphate positive electrode material particles to maintain unobstructed lithium ion transmission channels during charging and discharging under extreme environments, shortening the diffusion path of lithium ions, which greatly improves the lithium ion deintercalation / embedding efficiency and the electronic conductivity of the material, thereby reducing the low-temperature charge transfer impedance and pole sheet resistivity of the lithium iron phosphate positive electrode material, reducing polarization phenomena, and improving the electrochemical performance under extreme conditions.

[0152] (2) It can be seen from Table 1 and Table 5 that when the variance value S of the crystal grain size of the crystal characteristic growth surface of the lithium iron phosphate positive electrode material is 2 (D) The average grain size value D of all crystal directions of the material crystal a and D

[200] / D are within the specified range (S 2 (D): 5~25nm 2 ;D a :30-100nm; D

[200] : 15-85nm; D

[200] / D a :0.6~0.9), and the lithium iron phosphate cathode material also meets the crystal parameter factor Q at 0.03nm -2 ~0.15nm -2 Range value, lithium iron phosphate positive electrode material has lower low-temperature charge impedance and electrode resistivity, as well as better electrochemical performance.

[0153] (3) According to Table 5, the S 2 (D), D

[200] and Da values ​​are within the specified parameter range, and the electrochemical performance of the corresponding phosphoric acid positive electrode material is improved relative to the electrochemical performance of the phosphoric acid positive electrode material of Comparative Example 1. 2 (D), D

[200] , D a The parameter range is not specified in the embodiments of the present invention.

[0154] (4) According to Table 1 and Table 5, the S 2 (D), D

[200] , Da and D

[200] / Da values ​​are all within the specified parameter range, but the obtained crystal parameter factor Q is not within the range provided in the embodiments of the present invention. Compared with the lithium iron phosphate positive electrode material of Example 2, the lithium iron phosphate positive electrode material obtained in Example 3 has improved electrochemical performance and reduced battery internal resistance.

[0155] (5) According to Table 1 and Table 5, the S 2 (D), D

[200] , D a and D

[200] / D a The values ​​are all within the specified parameter range, and the parameter factor Q is also within the range provided by the embodiment of the present invention. Compared with Examples 2 and 3, several performance indicators of the corresponding lithium iron phosphate positive electrode material are also significantly improved.

[0156] (6) According to Table 1 and Table 5, by comparing Examples 1-8 with Comparative Example 1, the S 2 (D), D

[200] , D a It is not within the specified parameter range. Although its D

[200] / Da value meets the specified range, the extreme environment electrochemical performance of the obtained positive electrode material is still poor and the battery internal resistance is high.

[0157] It can be seen that when S 2 (D), D

[200] , D a and D

[200] / D a The four variables meet the value ranges specified in the embodiment of the present invention, and the crystal parameter factors are also within the preferred range. The performance indicators of the obtained lithium iron phosphate cathode material are the best, and at least S 2 (D), D

[200] and D a The low-temperature electrochemical performance and high-rate charge and discharge performance of the lithium iron phosphate positive electrode material formed by these three variables will also be improved.

[0158] Table 6 Test results of lithium iron phosphate positive electrode materials of Examples 9-13 and Comparative Examples 2-4

[0159]

[0160] (1) From the comprehensive data of Table 2 and Table 6, it can be seen that from the data of Example 1 and Comparative Examples 3-4, when no plasma jet is used and no polymer additive is added during the preparation process, the variance value S of the crystal characteristic growth crystal grain size of the materials in Comparative Examples 3 and 4 is 2 (D) is large, indicating that the growth rate of its crystal in all directions is uneven and anisotropic, the grain shape is irregular, and the obtained crystal parameter factor Q value is also small. The irregular grain morphology makes the internal structure of the material crystal tend to be disordered, which is not conducive to the diffusion and transfer of lithium ions. Therefore, the battery internal resistance of the lithium iron phosphate positive electrode material is high, and the low-temperature electrical performance and high-rate electrical performance are both poor.

[0161] (2) Compared with Example 1, the plasma treatment time of Example 9 is longer, and the concentration and dosage of the polymer additive are higher, which is conducive to promoting the primary nucleation of lithium iron phosphate and regulating its crystal structure, thereby obtaining grains with regular morphology and increasing the contact area between particles to improve the migration efficiency of lithium ions and the electronic conductivity of the material, thereby enhancing the extreme electrical properties of the positive electrode material.

[0162] (3) As can be seen from Table 2 and Table 6, the data of Example 1 and Comparative Examples 2 and 4 show that when no organic phosphonic acid is used in the preparation process and high-speed stirring and ultrasonic treatment are not performed, the average grain size D of the crystals of the materials of Comparative Examples 2 and 4 in all crystal directions is a Larger, D

[200] / D a The smaller the value, the longer the diffusion path of lithium ions in the material crystals, and the lower the exposure of the low surface energy

[200] crystal plane in the material crystals. This is not only not conducive to the increase of the specific surface area and compaction density of the lithium iron phosphate cathode material particles, but also, to a certain extent, easily leads to stress and surface lattice defects in the lithium iron phosphate cathode material during the lithium ion deintercalation / embedding process, thereby hindering the migration of lithium ions, slowing down the diffusion rate, and increasing the low-temperature charge transfer impedance and electrode resistance. The above factors lead to the poor low-temperature adaptability of the lithium iron phosphate cathode material and the reduction of its electrochemical performance under extreme environments.

[0163] (4) Compared with Example 1, Example 10 increases the high-speed stirring rate and prolongs the ultrasonic treatment time, which is conducive to obtaining lithium iron phosphate crystals with small grain size and material particles with a larger specific surface area, further promoting the diffusion of lithium ions and the improvement of the electronic conductivity of the material; and Example 11 uses hydroxyphosphonoacetic acid as the organic phosphonic acid. Compared with the single chelating group (amino group) in Example 1, the chelating group of the organic phosphine source has both hydroxyl and carboxyl groups. This structural compound is easier to adsorb onto the surface of the lithium iron phosphate primary particles to adjust its crystal structure, reduce the surface energy of the

[200] crystal plane, slow down its growth rate, increase the exposure of the crystal plane in the crystal, and thus reduce the local lattice defects generated when the material is charged and discharged in extreme environments, promote the conduction of lithium ions, and improve the electrochemical performance of the positive electrode material.

[0164] (5) Compared with Example 1, the extreme environment electrochemical performance of the lithium iron phosphate positive electrode material obtained by changing the hydrothermal reaction temperature in Example 12 and by changing the calcination temperature in Example 13 is similar to that of Example 1. This is related to the fact that the three have crystal parameter factors with similar numerical values. In addition, within a certain temperature range, a higher calcination temperature is conducive to the formation of a stable, continuous and tight carbon coating layer structure, thereby improving the electronic conductivity of the material.

[0165] Test Example 2

[0166] The lithium iron phosphate cathode materials prepared in Example 1 and Comparative Example 2 were subjected to powder diffraction testing, and the lithium iron phosphate cathode materials prepared in Example 1 and Comparative Example 4 were subjected to scanning electron microscopy testing. Figures 1 to 3 .

[0167] in, Figure 1 The XRD patterns of the lithium iron phosphate cathode materials prepared in Example 1 and Comparative Example 2 are as follows: Figure 2 This is the SEM image of the lithium iron phosphate cathode material prepared in Example 1. Figure 3 This is the SEM image of the lithium iron phosphate positive electrode material prepared in Comparative Example 4.

[0168] according to Figure 1 It can be seen from the XRD diagram that the diffraction curves of Example 1 and Comparative Example 2 correspond to the standard card of the lithium iron phosphate positive electrode material, and compared with Example 1, the diffraction peak intensity of the lithium iron phosphate positive electrode material of Comparative Example 2 at the

[200] crystal plane is weaker, indicating that the crystal plane is less exposed in the material crystal. Figure 2-3 The SEM image shows that the particles of the lithium iron phosphate positive electrode material prepared in Example 1 are regular in shape and evenly distributed, while the particles of the lithium iron phosphate positive electrode material prepared in Comparative Example 4 are irregular in shape and severely agglomerated, which is not conducive to improving the electrochemical performance of the material in extreme environments.

[0169] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: Lithium iron phosphate cathode material crystals meet the following requirements: (1) , 5nm 2 ≤S 2 (D) ≤25nm 2 ; Among them, S 2 (D) represents the grain size variance of the characteristic growth crystal plane of the lithium iron phosphate positive electrode material crystal, in nm 2 ; N is 7, indicating the seven characteristic growth crystal planes of [101], [111], [211], [311], [301], [121] and [040] of the lithium iron phosphate positive electrode material crystal; D i represents the grain size of the i-th characteristic growth crystal plane of the lithium iron phosphate positive electrode material crystal, in nm; represents the average value of the grain size of the seven characteristic growth crystal planes of the lithium iron phosphate positive electrode material crystal, in nm; (2) 15 nm ≤ D[200] ≤ 85 nm; D[200] represents the grain size of the [200] crystal plane of the lithium iron phosphate positive electrode material crystal, in nm; (3) 30nm≤D a ≤100nm;D a It represents the average grain size of the lithium iron phosphate positive electrode material crystals in all crystal directions, in nm.

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that 0.6≤ ≤0.9, It represents the ratio of the grain size of the [200] crystal plane of the lithium iron phosphate positive electrode material crystal to the average grain size of all crystal directions of the lithium iron phosphate positive electrode material crystal.

3. The lithium iron phosphate cathode material according to claim 1 or 2, characterized in that , 0.03nm -2 ≤Q≤0.15nm -2 ; Wherein, Q represents the crystal parameter factor of the lithium iron phosphate positive electrode material, the unit is nm -2 .

4. The lithium iron phosphate positive electrode material according to claim 1 or 2, characterized in that The lithium iron phosphate positive electrode material comprises a lithium iron phosphate matrix and a carbon coating layer coated on the surface of the lithium iron phosphate matrix; The general formula of the lithium iron phosphate matrix is ​​as follows: Li 1-x A x Fe 1-y M y (PO 4-m )D m ; wherein A is selected from at least one of Na and Mg; M is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D is selected from at least one of F and S; 0≤x≤0.1; 0≤y≤0.1 and 0≤m≤0.1; The content of the carbon coating layer coated on the surface of the lithium iron phosphate substrate is 1 wt % to 5 wt %.

5. A method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: include: A plasma jet is used to heat a reaction solution containing phosphorus, iron and lithium elements; The precursor solution formed by the heat treatment and the polymer additive solution are stirred at high speed and ultrasonically treated, and then a hydrothermal reaction is carried out. After the reaction is completed, the solution is dried to obtain a lithium iron phosphate precursor; The lithium iron phosphate precursor is mixed with a carbon source and then sintered to obtain a lithium iron phosphate positive electrode material; The plasma jet conditions include: an input voltage of 1000-1200 V, a processing frequency of 15-20 kHz, an argon flow rate of 1.5-2 L / min, a distance between the plasma jet nozzle and the surface of the reaction solution of 5-8 cm; a plasma jet processing time for the reaction solution of 30-50 min; and a heating temperature of 70-80° C.

6. The preparation method according to claim 5, characterized in that The reaction solution further includes M element and / or A element; The phosphorus element is derived from an organic phosphonic acid; the iron element is derived from a ferrous salt; the M element is derived from a water-soluble salt of the M element; the lithium element is derived from a lithium salt; and the A element is derived from a salt containing the A element. The ratio of the total molar number of the iron element and the M element, the total molar number of the lithium element and the A element, and the molar number of the phosphorus element is 1:1:(0.95-1.1); The molar ratio of the lithium element to the A element is (0.9-1): (0-0.1); The molar ratio of the iron element to the M element is (0.9-1): (0-0.1); The organic phosphonic acid is selected from any one of aminotrimethylphosphonic acid, hydroxyethyldiphosphonic acid and hydroxyphosphonoacetic acid.

7. The preparation method according to claim 5, characterized in that The polymer additive is selected from any one of hydroxypropyl methylcellulose, methylcellulose and polyvinyl pyrrolidone; The carbon source is selected from carbohydrate compounds; When mixed with the carbon source, it also includes adding a D source, wherein the D source is selected from compounds containing D elements; The mass concentration of the polymer additive solution is 0.5wt%-1.0wt%; The molar ratio of the iron element, the carbon source and the D source is 1: (0.05-0.15): (0-0.1).

8. The preparation method according to claim 5 or 7, characterized in that The high-speed stirring rate is 600-1000 r / min; the ultrasonic treatment time is 20-30 min; The conditions of the hydrothermal reaction include: a temperature of 150-180°C and a time of 8-10 hours; The sintering process includes: holding at 400-550°C for 2-5 hours, and then firing at 600-750°C for 6-8 hours.

9. A lithium-ion battery, characterized in that: It includes the lithium iron phosphate positive electrode material according to claim 1.

Images