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

Abstract

The invention relates to the technical field of lithium ion battery materials, in particular to a lithium iron phosphate positive electrode material, a preparation method thereof and a lithium ion battery. The lithium iron phosphate positive electrode material crystal meets the following requirements: (1) # imgabs0 #, S2 (D) is greater than or equal to 5nm < 2 > and less than or equal to 25nm < 2 >; (2) 15 nm < = D [200] < = 85 nm; (3) 30 nm < = Da < = 100 nm; wherein S2 (D) represents the grain size variance of the characteristic growth crystal face of the lithium iron phosphate positive electrode material crystal; d [200] represents the grain size of the [200] crystal face of the lithium iron phosphate positive electrode material crystal; da represents the average grain size of all crystal orientations of the lithium iron phosphate positive electrode material crystal. The lithium ion battery prepared from the lithium iron phosphate positive electrode material has good low-temperature electrochemical performance and high-rate charge-discharge performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery materials, and more specifically, to a lithium iron phosphate cathode material, 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 (LiFePO 4 ) is used as the cathode 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 cathode material formed by lithium iron phosphate has defects such as 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 longer diffusion path due to the longer diffusion path when lithium ions are intercalated / deintercalated between the two-phase interfaces of LiFePO 4 / FePO 4 result in a lower diffusion rate, so that the active material of the lithium iron phosphate cathode material is not fully utilized during high-current charge and discharge, and the internal resistance and polarization of the electrode increase, resulting in rapid capacity decay during high-rate charge and discharge. At the same time, compared with other cathode materials, the main reason for the unsatisfactory low-temperature electrical performance of the lithium iron phosphate cathode material 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 it difficult for lithium ions to intercalate / deintercalate, and the internal resistance and polarization phenomenon of the battery will also increase. Therefore, the low-temperature performance of lithium batteries is poor. Moreover, under extreme overcharge or low-temperature conditions, it will also cause local lattice damage inside the lithium iron phosphate cathode material to a certain extent, introducing defects and affecting the crystal order, which will also affect the intercalation / deintercalation kinetic rate of lithium ions.

[0004] In view of the above problems, a series of methods are currently used to improve the lithium iron phosphate cathode material, such as carbon coating, ion doping, and particle nanosizing. Although the above optimization methods have achieved certain effects, with the continuous expansion of the application field 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 specifically proposed. Summary of the Invention

[0006] The purpose of the present invention is to provide a lithium iron phosphate cathode material, a preparation method thereof, and a lithium-ion battery. An embodiment of the present invention provides a lithium iron phosphate cathode material, and the lithium-ion battery formed thereby has good low-temperature electrochemical performance and high-rate charge and discharge performance.

[0007] The present invention is implemented in the following manner: The present invention provides a lithium iron phosphate cathode material, and the crystal of the lithium iron phosphate cathode material meets the following requirements: (1) , 5 nm 2 ≤ S 2 (D) ≤ 25 nm 2 ; wherein, S 2 (D) represents the grain size variance of the characteristic growth crystal plane of the lithium iron phosphate cathode material crystal, with the unit of nm 2 ; N is 7, indicating 7 characteristic growth crystal planes of the lithium iron phosphate cathode material crystal, namely

[101] ,

[111] ,

[211] ,

[311] ,

[301] ,

[121] and

[040] ; D i represents the grain size of the i-th characteristic growth crystal plane of the lithium iron phosphate cathode material crystal, with the unit of nm; represents the average value of the grain sizes of the 7 characteristic growth crystal planes of the lithium iron phosphate cathode material crystal, with the unit of nm; (2) 15 nm ≤ D

[200] ≤ 85 nm; D

[200] represents the grain size of the

[200] crystal plane of the lithium iron phosphate cathode material crystal, with the unit of nm; (3) 30 nm ≤ D a ≤ 100 nm; D a represents the average grain size of all crystal orientations of the lithium iron phosphate cathode material crystal, with the unit of nm.

[0008] In an alternative embodiment, 0.6 ≤ ≤ 0.9, represents the ratio of the grain size of the

[200] crystal plane of the lithium iron phosphate cathode material crystal to the average grain size of all crystal orientations of the lithium iron phosphate cathode material crystal.

[0009] In an alternative embodiment, , 0.03 nm -2 ≤ Q ≤ 0.15 nm -2 ; wherein, Q represents the crystal parameter factor of the lithium iron phosphate cathode material, with the unit of nm -2 .

[0010] In an alternative embodiment, the lithium iron phosphate cathode material includes a lithium iron phosphate matrix and a carbon coating layer coated on the surface of the lithium iron phosphate matrix; wherein, 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 matrix is 1 wt% - 5 wt%.

[0011] In a second aspect, the present invention provides a method for preparing the lithium iron phosphate cathode material described in the foregoing embodiment, including: heating a reaction solution containing phosphorus element, iron element and lithium element by using a plasma jet; High-speed stirring and ultrasonic treatment are performed on the precursor solution formed by the heating treatment and the polymer additive solution, and then a hydrothermal reaction is carried out. After the reaction is completed, drying is performed to obtain a lithium iron phosphate precursor; The lithium iron phosphate precursor is mixed with a carbon source and then sintered to obtain the lithium iron phosphate cathode material.

[0012] In an optional embodiment, the conditions of the plasma jet include: the input voltage is 1000 - 1200 V, the processing frequency is 15 - 20 kHz, the argon gas flow rate is 1.5 - 2 L / min, the distance between the nozzle of the plasma jet and the surface of the reaction solution is 5 - 8 cm; the processing time of the plasma jet on the reaction solution is 30 - 50 min; The heating temperature is 70 - 80 °C.

[0013] In an optional embodiment, the reaction solution further includes M element and / or A element; The phosphorus element is derived from organic phosphonic acid; the iron element is derived from ferrous salt; the M element is derived from a water-soluble salt of M element; the lithium element is derived from lithium salt; the A element is derived from a salt containing 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 to 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 hydroxymethylenephosphonic acid.

[0014] In an optional embodiment, the polymer additive is selected from any one of hydroxypropyl methylcellulose, methylcellulose and polyvinylpyrrolidone; The carbon source is selected from saccharide compounds; When mixing with a carbon source, it further includes adding a D source, and the D source is selected from compounds containing D element; 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).

[0015] In an optional embodiment, the rate of high-speed stirring is 600 - 1000 r / min; the time of ultrasonic treatment is 20 - 30 min; The conditions of the hydrothermal reaction include: the temperature is 150 - 180 °C and the time is 8 - 10 hours; Sintering includes: heat preservation treatment at 400 - 550 °C for 2 - 5 hours, and then roasting at 600 - 750 °C for 6 - 8 hours.

[0016] In a third aspect, the present invention provides a lithium-ion battery, which includes the lithium iron phosphate cathode material described in the foregoing embodiments.

[0017] The present invention has the following beneficial effects: (1) The structure inside the crystal of the lithium iron phosphate cathode material provided by the embodiments of the present invention is more ordered, its lithium-ion diffusion path is more direct and effective, the diffusion resistance of lithium ions is reduced and its migration rate is increased, which are all beneficial to reducing the polarization phenomenon during the charge and discharge process of the lithium iron phosphate battery.

[0018] (2) The lithium iron phosphate cathode material provided by the embodiments of the present invention has regularly shaped grains, and the surface of the regularly shaped grains is more uniform and smooth, which is beneficial to increasing the contact area between particles and helps to form a continuous electron conduction network, thereby improving the conductivity of the lithium iron phosphate cathode material.

[0019] (3) The surface defects and stress of the lithium iron phosphate cathode material provided by the embodiments of the present invention are less. Under extreme charge and discharge conditions, it can inhibit the generation of local lattice defects in the lithium iron phosphate cathode material, reduce the stress generated during the lithium-ion deintercalation / insertion process, and thus reduce the migration internal resistance of lithium ions.

[0020] (4) In the lithium iron phosphate cathode material provided by the embodiments of the present invention, the diffusion distance of lithium ions inside the crystal of the cathode material is short, the migration rate during the deintercalation / insertion process is fast, the polarization generated by the electrode is small, and then the lithium-ion battery can be charged and discharged under high-rate conditions, and the lithium battery has low-temperature adaptability.

[0021] (5)The preparation method provided by the embodiments 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 beneficial to the formation of regular crystals and their orderly arrangement, and can eliminate the internal stress of the lithium iron phosphate cathode material and improve the crystallinity and conductivity of the material, thereby ensuring the performance of the lithium iron phosphate cathode material. Description of the Drawings

[0022] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention, and thus should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0023] Figure 1 XRD diagram of the lithium iron phosphate cathode materials prepared in Example 1 and Comparative Example 2; Figure 2 SEM diagram of the lithium iron phosphate cathode material prepared in Example 1; Figure 3 SEM diagram of the lithium iron phosphate cathode material prepared in Comparative Example 4. Detailed Embodiments

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. For those conditions not specified in the embodiments, they shall be carried out according to conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments whose manufacturers are not indicated, they are all conventional products that can be obtained through commercial purchase.

[0025] The embodiments of the present invention provide a lithium iron phosphate cathode material, and a battery containing this lithium iron phosphate cathode material has good low-temperature electrochemical performance and high-rate charge and discharge performance.

[0026] Specifically, the crystals of this lithium iron phosphate cathode material meet the following requirements: (1)5nm 2 ≤S 2 (D) ≤25nm 2 ; for example, it is 5 nm 2 、10 nm 2 、15 nm 2 、20 nm 2 、25 nm 2 or any value between 5 - 25nm 2 Among them, S 2(D) represents the variance of the grain size of the characteristic growth crystal plane of the lithium iron phosphate cathode material crystal, and its unit is nm 2 ; S 2 (D) reflects the dispersion degree of the grain sizes corresponding to different growth crystal planes of the crystal 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, grains with regular shapes mean that the internal structure of the crystal is more ordered, which helps to form a more direct and effective lithium ion diffusion path, reduce the diffusion resistance of lithium ions, increase their migration rate, and reduce the polarization phenomenon during charge and discharge; moreover, the surfaces of grains with regular shapes are more uniform and smooth, which is conducive to increasing the contact area between particles and helps to form a continuous electron conduction network, improving the conductivity of the lithium iron phosphate cathode material.

[0027] Specifically, S 2 (D) The smaller the value, the more regular the grain shape, indicating that the growth rates of the crystal in the lithium iron phosphate cathode material in the directions of each growth crystal plane are more uniform, its grain size is basically isotropic, and the internal structure of the material crystal tends to be an ordered state; while S 2 (D) The larger the value, it indicates that the growth rates of the crystal in the lithium iron phosphate cathode material in all directions are not uniform, showing anisotropy, the obtained 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, the smaller S 2 (D) is, the better, but it is not easy to achieve when S 2 (D) is too small. That is to say, in the embodiments of the present invention, it is not easy to achieve when S 2 (D) is less than 5 nm 2 ; and when S 2 (D) is greater than 25 nm 2 , the low-temperature electrochemical performance and high-rate charge and discharge performance of the lithium ion battery formed by the lithium iron phosphate cathode material cannot be effectively improved.

[0028] Furthermore, the calculation method of S 2 (D) is as follows: , where the value of N is 7, representing the following 7 characteristic growth crystal planes of the lithium iron phosphate cathode material crystal, which are

[101] ,

[111] ,

[211] ,

[311] ,

[301] ,

[121] and

[040] crystal planes respectively. D i represents the grain size of the i-th characteristic growth crystal plane of the lithium iron phosphate cathode material crystal, and its unit is nm; represents the average value of the grain sizes of the 7 characteristic growth crystal planes of the lithium iron phosphate cathode material crystal, and its unit is nm; It should be noted that: (1) The corresponding relationship between the main diffraction peak positions of lithium iron phosphate XRD and crystal planes: 20.85°

[101] , 25.63°

[111] , 29.61°

[211] , 35.67°

[311] , 32.26°

[301] , 36.53°

[121] , and 61.77°

[040] . (2) 15 nm ≤ D

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

[200] represents the grain size of the

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

[200] crystal plane has a relatively low surface energy, which is the dominant crystal plane for reducing surface defects and stress of the lithium iron phosphate cathode material. Under extreme charge and discharge conditions, the existence of this crystal plane can not only help inhibit the possible local lattice defects generated in the lithium iron phosphate cathode material, but also contribute to reducing the stress generated during the lithium ion deintercalation / insertion process, and to a certain extent, can reduce the migration internal resistance of lithium ions. By adopting D

[200] within the above range in the embodiments of the present invention, the performance of the lithium iron phosphate cathode material can be improved. It is difficult to achieve D

[200] lower than 15 nm in the embodiments of the present invention, and if D

[200] is greater than 85 nm, it will lead to a decrease in the performance of the formed lithium iron phosphate cathode material, and then result in poor low-temperature electrochemical performance or high-rate charge and discharge performance of the formed lithium ion battery.

[0029] (3) 30 nm ≤ D a ≤ 100 nm; for example, it can be 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any value between 30 - 100 nm. D a represents the average grain size of all crystal orientations of the lithium iron phosphate cathode material crystal, and its unit is nm. D a 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 inside the crystal of the lithium iron phosphate cathode material, the faster the migration rate during its deintercalation / insertion process, and the smaller the polarization generated by the electrode, which is beneficial for the formed lithium ion battery to perform charge and discharge under high-rate conditions and its low-temperature adaptability; moreover, small grain sizes are often accompanied by a larger specific surface area, and a large surface is conducive to the uniform distribution of the subsequent 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. If D a is less than 30 nm, it is not easy to achieve, and if it is greater than 100 nm, it will result in poor low-temperature electrochemical performance or high-rate charge and discharge performance of the formed lithium ion battery.

[0030] When the lithium iron phosphate cathode material provided by the embodiment of the present invention meets the above three requirements at the same time, the formed lithium ion battery has good low-temperature electrochemical performance and high-rate charge and discharge performance.

[0031] It should be noted that the different characteristic growth crystal planes of the above lithium iron phosphate cathode material crystal and the grain size of the

[200] crystal plane are obtained by calculating through the Scherrer formula after the full-spectrum fitting of the X-ray diffraction curve of the lithium iron phosphate cathode material by MDI Jade; the average grain size of all crystal orientations of the lithium iron phosphate cathode material crystal is obtained by calculating through the Cauchy distribution formula after the full-spectrum fitting of the X-ray diffraction curve of the lithium iron phosphate cathode material by MDI Jade.

[0032] In addition to meeting S 2 (D), D

[200] and D a requirements, if further meeting the requirement of 0.6 ≤ ≤ 0.9, for example, being 0.6, 0.7, 0.8, 0.9 or any value between 0.6 - 0.9, the electrochemical performance of the obtained lithium iron phosphate cathode material under extreme conditions is improved.

[0033] Specifically, represents the ratio of the grain size of the

[200] crystal plane of the lithium iron phosphate cathode material crystal to the average grain size of all crystal orientations of the lithium iron phosphate cathode material crystal, 0.6 ≤ ≤ 0.9. It can reflect the exposure degree of the

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

[200] / D a 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 / insertion process, and the conduction of lithium ions is blocked; the larger the D

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

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

[200] and Da need to be controlled within a suitable and balanced range, too high a D

[200] / D a value is not easy to achieve, and too low a D

[200] / D a will lead to a reduction in the performance of the lithium iron phosphate cathode material.

[0034] Furthermore, in addition to meeting S 2 (D), D

[200] , D a and requirements, if further meeting the requirement of 0.03nm -2 ≤ ≤0.15 nm -2 , such as 0.03 nm 2 , 0.04 nm 2 , 0.05 nm 2 , 0.06 nm 2 , 0.07 nm 2 , 0.08 nm 2 , 0.09 nm 2 , 0.10 nm 2 , 0.11 nm 2 , 0.12 nm 2 , 0.13 nm 2 , 0.14 nm 2 , 0.15 nm 2 or any value between 0.03 - 0.15 nm, and then the obtained lithium iron phosphate cathode material has better high-rate electrical properties and low-temperature electrical properties. 2 Specifically, the crystal parameter factor of the lithium iron phosphate cathode material obtained from the above relationship is denoted as Q, and its unit is nm.

[0035] Specifically, the crystal parameter factor of the lithium iron phosphate cathode material obtained from the above relationship is denoted as Q, and its unit is nm -2 , and this index is related to the grain size variance value 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 orientations of the lithium iron phosphate cathode material crystal a The corresponding relationship can comprehensively reflect the electrochemical performance of the lithium iron phosphate cathode material. Generally, the smaller the S 2 (D), D a value, and the larger the D

[200] / Da value, the larger the crystal parameter factor Q value of the lithium iron phosphate cathode material, that is, the smoother the lithium ion transport channel during the charge and discharge process of the lithium iron phosphate cathode material in an extreme environment, the higher the electronic conductivity of the lithium iron phosphate cathode material, and the corresponding reduction of the battery internal resistance and polarization phenomenon; further, the electrochemical performance of the lithium iron phosphate battery will also be improved. Theoretically, the larger the crystal parameter factor Q value, the better, but it is not easy to achieve technically when the Q value is too large, and the performance of the lithium iron phosphate cathode material will decrease when the Q value is too small.

[0036] In summary, when the lithium iron phosphate cathode material simultaneously meets the conditions of S 2 (D), D

[200] , D a , and Q, the performance of the lithium iron phosphate cathode material is the best, and the lithium ion battery prepared from this cathode material has more excellent low-temperature electrochemical performance and high-rate charge and discharge performance.

[0037] Further, the lithium iron phosphate cathode material includes a lithium iron phosphate matrix and a carbon coating layer coated on the surface of the lithium iron phosphate matrix. Among them, 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 matrix is 1 wt% to 5 wt%. For example, it is 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt% or any value between 1-5 wt%.

[0038] In a second aspect, the present invention provides a method for preparing the lithium iron phosphate cathode material described in the foregoing embodiment, including: S1. Form a reaction solution; Respectively disperse the raw materials containing iron elements and lithium elements into deionized water. For example, disperse the ferrous salt containing iron elements and the lithium source containing lithium elements into deionized water, and blow inert gas (such as including but not limited to nitrogen) under stirring conditions to remove the dissolved oxygen in the solution to obtain a metal salt solution.

[0039] The metal salt solution may further include M elements and / or A elements. For example, disperse the ferrous salt, the lithium source, the M source containing M elements, and / or the A source containing A elements into the above-mentioned deionized water, and blow inert gas under stirring conditions to obtain a metal salt solution.

[0040] Disperse the raw material containing phosphorus element, that is, the phosphorus source, into an alcohol solvent (such as including but not limited to ethanol) to obtain a phosphorus source solution.

[0041] Mix and stir the above metal salt solution and phosphorus source solution in an atmosphere of a protective gas (such as including but not limited to nitrogen) to obtain a mixed solution, and then adjust the pH of the mixed solution to 6-8 to form a reaction solution. It can be seen that the reaction solution contains phosphorus elements, iron elements, and lithium elements, and may also contain M elements and / or A elements.

[0042] Among them, the phosphorus element is derived from organic phosphonic acids, such as, including but not limited to, any one of aminotrimethylphosphonic acid, hydroxyethyldiphosphonic acid, and hydroxyphosphinylacetic acid. The iron element is derived from ferrous salts, such as, including but not limited to, any one of ferrous oxalate, ferrous chloride, and ferrous acetate. The M element is derived from water-soluble salts of the M element, specifically water-soluble salts of elements Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, and Y. The A element is derived from salts containing the A element, such as, including but not limited to, any one of sodium fluoride, magnesium fluoride, sodium chloride, magnesium chloride, and sodium sulfide. The lithium element is derived from lithium salts, such as, including but not limited to, any one of lithium oxalate, lithium chloride, and lithium acetate.

[0043] Furthermore, the amounts of the raw materials containing the above elements are fed according to the ratio of 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); the molar ratio of the iron element to the M element is (0.9 - 1):(0 - 0.1).

[0044] S2. Heat treatment; The above reaction solution is heat-treated by using a plasma jet.

[0045] The plasma jet is a mature plasma generation technology. When the high-energy particles in the discharge filament in the device bombard the mixed solution of metal salts and organic phosphonic acids, it will give them higher energy, which is conducive to promoting the rapid nucleation of the crystals of the lithium iron phosphate cathode material and forming primary crystal nuclei, thereby shortening the crystallization time in the hydrothermal reaction process. Moreover, the plasma will also generate a large number of highly active groups, which will trigger the self-regulation effect of the reaction system and reduce the supersaturation of the solution, which is conducive to the regulation of 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 be used as the phosphorus source for synthesizing lithium iron phosphate, but also be adsorbed onto the primary particles of the lithium iron phosphate cathode material as a chelating agent (coordination effect of ferrous ions with hydroxyl, amino, or carboxyl groups) to adjust the surface structure of the crystals, further reducing the surface energy of the

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

[200] crystal plane and will not be quickly consumed, so that the crystals of the lithium iron phosphate cathode material can obtain more

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

[0046] The plasma technology device for implementing the above plasma jet is an existing device. The following is an example of the processing process of the plasma technology device in the embodiment of the present invention: Pour the reaction solution into a conical flask and place it in a water bath, keep continuous stirring, and a condenser is connected to the conical flask to prevent the evaporation of the solvent. The plasma experimental device consists of a plasma generator, a plasma nozzle, an argon gas source delivery system, etc. During the reaction process, the plasma generator applies a high-voltage electric field, and the atoms in the introduced gas are ionized to generate positive ions and free electrons, thereby forming a low-temperature plasma. Then, the atmospheric pressure spray gun uses the interaction between the electric field and the air flow to spray the low-temperature plasma from the nozzle onto the surface of the reaction solution in the conical flask for reaction.

[0047] Therefore, the conditions for the plasma jet in the embodiment of the present invention include: the input voltage is 1000 - 1200V, such as 1000V, 1100V, 1200V or any value between 1000 - 1200V. The processing frequency is 15 - 20kHz, such as 15kHz, 16kHz, 17kHz, 18kHz, 19kHz, 20kHz or any value between 15 - 20kHz. The argon gas flow rate is 1.5 - 2L / min, such as 1.5L / min, 1.6L / min, 1.7L / min, 1.8L / min, 1.9L / min, 2.0L / min or any value between 1.5 - 2.0L / min. The distance between the nozzle of the plasma jet and the surface of the reaction solution is 5 - 8cm; such as 5cm, 6cm, 7cm, 8cm or any value between 5 - 8cm. The processing time of the plasma jet on the reaction solution is 30 - 50min; such as 30min, 35min, 40min, 45min, 50min or any value between 30 - 50min.

[0048] Adopting the above conditions is beneficial to heat treatment and can make the grain size of the lithium iron phosphate cathode material isotropic. If other heat treatment methods are used to treat the reaction solution, such as conventional water bath heating, then the grain size of the lithium iron phosphate cathode material cannot be made isotropic, and the formed battery cannot meet the low-temperature electrochemical performance and high-rate charge and discharge performance.

[0049] The temperature of the above heat treatment is 70 - 80°C, such as 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C, 80°C or any value between 70 - 80°C.

[0050] S3. Form a lithium iron phosphate precursor; The precursor solution formed by the above heat treatment is stirred at high speed and ultrasonically treated with a polymer additive solution.

[0051] Specifically, in the embodiments of the present invention, the polymer additive is selected from any one of hydroxypropyl methylcellulose, methylcellulose, and polyvinylpyrrolidone. Using the above polymer additive is more conducive to realizing its functions as a bridging agent and delaying the diffusion of solutes.

[0052] 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, it is 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%.

[0053] It should be noted that the solvent for dissolving the polymer additive is a solvent that can dissolve the polymer additive as recorded in the prior art, and it can be water or other solvents. When both water and other solvents can dissolve it, water is preferably used for dissolution. The viscosity of the polymer additive will increase with the increase of its mass concentration. Using the polymer additive with the above mass concentration can delay the diffusion of solutes, and can promote local crystal nucleation acceleration and precipitation of a large number of fine crystals.

[0054] For the dosage of the polymer additive solution, it can be adjusted according to production requirements. For example, in the embodiments of the present invention, 5 - 10 ml of the polymer additive solution is added corresponding to every 500 ml of the precursor solution.

[0055] Furthermore, in the embodiments of the present invention, high-speed stirring and ultrasonic treatment can reduce the crystal size of primary crystal nuclei to promote the formation of subsequent small particles. Specifically, the rate of high-speed stirring is 600 - 1000 r / min; for example, it is 600 r / min, 700 r / min, 800 r / min, 900 r / min, 1000 r / min or any value between 600 - 1000 r / min. The time of ultrasonic treatment is 20 - 30 min; for example, it is 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.

[0056] After the ultrasonic treatment, the mixed solution is transferred to a stainless steel autoclave with a polytetrafluoroethylene liner and placed in an oven at 150°C - 180°C for heat preservation for 8 - 10 h. After the hydrothermal reaction, the product is cooled to room temperature and washed, for example, centrifugally washed successively with deionized water and absolute ethanol, and finally dried to obtain the lithium iron phosphate precursor; for example, dried in a vacuum oven at 70°C for 12 h.

[0057] During the hydrothermal reaction process, the polymer additive with a certain viscosity used in the embodiments of the present invention can delay the diffusion of solutes, thereby accelerating local nucleation of 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-shaped crystals and arrange them orderly, finally obtaining lithium iron phosphate precursor particles with good crystallinity.

[0058] S4. Form a lithium iron phosphate positive electrode material; Next, the lithium iron phosphate precursor is mixed and ball-milled with a carbon source. If the lithium iron phosphate positive electrode material contains element D, then at this time, the lithium iron phosphate precursor is mixed and ball-milled with a carbon source and a D source containing element D, wherein the molar ratio of the iron element, the carbon source, and the D source is 1:(0.05 - 0.15):(0 - 0.1).

[0059] It should be noted that the D source can be a sulfide or a fluoride, such as including but not limited to sodium sulfide, potassium sulfide, sodium fluoride, potassium fluoride, and ammonium fluoride.

[0060] Then sintering is carried out. Specifically, the sintering process includes: performing a heat preservation treatment at 400 - 550 °C for 2 - 5 hours, and then roasting at 600 - 750 °C for 6 - 8 hours.

[0061] In the embodiments of the present invention, the lithium iron phosphate precursor particles are ball-milled and mixed with a carbon source, placed in a muffle furnace, and subjected to low-temperature and high-temperature roasting successively to obtain a lithium iron phosphate positive electrode material, thereby eliminating the internal stress of the lithium iron phosphate positive electrode material and improving the crystallinity and conductivity of the lithium iron phosphate positive electrode material.

[0062] In a third aspect, the present invention provides a lithium-ion battery, which includes the lithium iron phosphate positive electrode material described in the foregoing embodiment.

[0063] The embodiments of the present invention adopt the following detection method to test relevant properties: 1. Use an X-ray diffractometer (XRD) to test the phase structure of the lithium iron phosphate positive electrode material, and then use MDI Jade to perform full-spectrum fitting on the obtained XRD data until the fitting error factor R ≤ 15%. After the fitting is completed, export the data, and use the Cauchy distribution formula and the Scherrer formula to calculate the average grain size of the lithium iron phosphate positive electrode material and the grain size of the crystal plane corresponding to a certain diffraction angle.

[0064] (1) Calculating the average grain size D of the lithium iron phosphate positive electrode material by the Jade-Cauchy step method a Cauchy distribution formula: (n, k values are taken as 1)

[0065] Among them, is half of the diffraction angle (2 ); is the full width at half maximum under the diffraction peak corresponding to a certain diffraction angle, with the unit of radian; is the wavelength of the incident X-ray. Generally, CuKα is selected, and its = 0.15406 nm; Da is the average grain size, with the unit of nm, represents the microstrain.

[0066] After fitting the original XRD data of the test sample with Jade (diffraction angle: 10° - 90°), substitute it into the above formula for processing. Use sinθ / as the abscissa and (FWHM×cosθ) / λ as the ordinate to make a scatter plot. Then, perform a linear fit on the scatter plot to obtain a fitting line and a fitting equation (a binary linear equation). According to the intercept 1 / Da value of the fitting equation, calculate the average grain size of the material.

[0067] (2) Calculate the grain size D of the crystal plane corresponding to a certain diffraction angle of the lithium iron phosphate cathode material using the Scherrer formula.

[0068]

[0069] Among them, k is a constant, generally taken as 0.89.

[0070] After fitting the original XRD data of the test sample with Jade, substitute it into the above formula for processing to calculate the grain size of the crystal plane corresponding to a certain diffraction angle (

[101] ,

[111] ,

[211] ,

[311] ,

[301] ,

[121] ,

[040] and

[200] crystal planes).

[0071] 2. Specific surface area test Use a Micromeritics ASAP2460 fully automatic specific surface area analyzer to measure the specific surface area of the powder particles. The data of the nitrogen adsorption - desorption curve in the medium - low pressure stage is analyzed and calculated through the BET formula to obtain the specific surface area of the material, with the unit of m 2 / g.

[0072] 3. Compaction density test Use a powder compaction density tester to measure the compaction density of the powder particles. The pressure is set to 3T, with the unit of g / cm 3 .

[0073] 4. Low - temperature charge - transfer impedance test Use an electrochemical workstation with a temperature control system and supporting EIS function to perform low-temperature charge transfer impedance tests on lithium-ion batteries with lithium iron phosphate materials as the positive electrode. Before the test, first pre-treat the sample to ensure the cleanliness of the electrode surface and sufficient electrolyte in the battery chamber. Then place the sample in a low-temperature environment, set the temperature to -30°C, and keep it for a period of time to make the sample temperature uniform and stable. Then start the impedance test. During the test, record the open-circuit voltage of the system, set the scanning frequency range to 0.01 Hz to 100 kHz, and use the default value of 0.005 V for the amplitude. Record the impedance data during the process. After the test, use software tools to analyze and fit the test data to extract the parameters related to the charge transfer process, namely the low-temperature charge transfer resistance R ct 。

[0074] 4. Resistivity of the electrode At room temperature (25°C), use the two-probe test method to measure the resistivity (Ω·cm) of the lithium iron phosphate positive electrode, and set the current intensity to 2 A.

[0075] 5. Electrochemical performance test Electrode preparation and button cell assembly: Uniformly mix lithium iron phosphate positive electrode material, conductive agent acetylene black, and adhesive polyvinylidene fluoride in a mass ratio of 92:4:4 in N-methylpyrrolidone to make a slurry, then coat it on aluminum foil and place it in a vacuum drying oven for drying. Then use a tablet press to press it into a positive electrode, the negative electrode is a lithium metal sheet, the electrolyte is 1 mol / L lithium hexafluorophosphate-ethylene carbonate: dimethyl carbonate (LiPF6-EC:DMC, volume ratio 1:1), and a polypropylene porous membrane is used as the separator. The battery is assembled in an argon glove box.

[0076] Battery rate test steps: Divide the prepared button cells 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, adjust the temperature of the test chamber to 25°C (normal temperature group) or -30°C (low temperature group), the charge and discharge voltage range is 2.5 - 4.5 V. First, charge at a constant current to 4.5 V, then discharge at a rate current to 2.5 V, and the discharged capacity is the discharge capacity at that rate. After the discharge, charge at a constant current to 4.5 V again; then perform the test at the next rate.

[0077] The features and performance of the present invention will be further described in detail below in conjunction with the embodiments.

[0078] Examples 1 - 13 Examples 1 - 13 and Comparative Examples 1 - 4 respectively provide a lithium iron phosphate positive electrode material, wherein, the S of the lithium iron phosphate positive electrode material 2 (D), D

[200] , D a 、 For the content such as Q, the content of the carbon coating layer, and the chemical formula of the lithium iron phosphate matrix, please refer to Table 1 and Table 2.

[0079] Table 1 Crystal parameters of the lithium iron phosphate cathode materials of Examples 1-8 and Comparative Example 1

[0080] Table 2 Crystal parameters of the lithium iron phosphate cathode materials of Examples 9-13 and Comparative Examples 2-4

[0081] Examples 1-13 and Comparative Examples 1-4 respectively provide preparation methods of the lithium iron phosphate cathode materials. Here, the preparation method of the lithium iron phosphate cathode material of Example 1 is taken as an example for illustration.

[0082] The preparation method of the lithium iron phosphate cathode material of Example 1 is as follows: S1. Weigh ferrous chloride and lithium oxalate and disperse them in deionized water. Bubble with nitrogen for 1 h under stirring conditions to remove the dissolved oxygen in the solution, and obtain a metal salt solution.

[0083] Weigh aminotrimethylphosphonic acid and disperse it in an ethanol solution to obtain a phosphorus source solution.

[0084] Stir the metal salt solution and the phosphorus source solution under the protection of a nitrogen atmosphere for 30 min to obtain a mixed solution. Subsequently, adjust the pH value of the obtained mixed solution to 7 with ammonia water to form a reaction solution, and then place it in a plasma jet device for water bath heating. Keep continuous stirring during the reaction process. After the reaction ends, obtain a precursor solution.

[0085] Among them, the treatment process of the plasma technology device: Pour the reaction solution into a conical flask and place it in a water bath. Keep continuous stirring. A condenser is connected to the conical flask; the plasma experimental device consists of a plasma generator, a plasma nozzle, an argon gas source delivery system, etc. Among them, the main technical parameters of the experimental device during the reaction process are: the input voltage is 1100 V, the treatment frequency is 15 kHz, the argon gas flow rate is 1.5 L / min, the distance between the nozzle and the surface of the reaction solution is 6 cm, the treatment time of the reaction solution by the plasma jet device is 35 min, and the temperature of the water bath heating is 75 °C. The molar ratio of iron element, lithium element, and phosphorus element is 1:1:1.

[0086] S2. Mix 7.5 mL of a hydroxypropyl methylcellulose solution with a mass concentration of 0.6 wt% and the 500 mL of the precursor solution obtained in step S1 under the protection of a nitrogen atmosphere at a high speed. The stirring rate is 750 r / min, and perform ultrasonic treatment for 25 min.

[0087] Subsequently, the precursor mixture was transferred to a stainless-steel autoclave lined with polytetrafluoroethylene and placed in an oven at 160 °C for 9 h. After the reaction, the product was cooled to room temperature and centrifugally washed successively with deionized water and absolute ethanol, and finally dried in a vacuum oven at 70 °C for 12 h to obtain the lithium iron phosphate precursor.

[0088] The lithium iron phosphate precursor and glucose were ball-milled for 1 h to obtain a mixed precursor, and then the well-ground precursor was placed in a muffle furnace filled with high-purity argon for sintering. It was heated to 450 °C at a rate of 5 °C / min and held for 4 h; then it was further heated to 650 °C and calcined for 6 h to obtain the lithium iron phosphate cathode material. Among them, the molar ratio of iron element in the lithium iron phosphate to glucose was 1:0.10.

[0089] The preparation methods of the lithium iron phosphate cathode materials in Examples 2-8 and Comparative Example 1 refer to the preparation method of Example 1, with the difference being only in some conditions. The specific conditions are shown in Tables 3 and 4.

[0090] Table 3 Specific conditions of the preparation method of the lithium iron phosphate cathode material (1)

[0091] Table 4 Specific conditions of the preparation method of the lithium iron phosphate cathode material (2)

[0092] The preparation methods of the lithium iron phosphate cathode materials provided in Examples 9-13 and Comparative Examples 2-4 also refer to the preparation method provided in Example 1, with only some operations being different, as follows: Example 9: Compared with Example 1, the time for treating the mixture with the plasma device in step S1 was changed to 45 min, the mass concentration of the polymer additive in S2 was 0.8 wt%, and the dosage was 8.0 mL, and the remaining steps remained unchanged.

[0093] Example 10: Compared with Example 1, the high-speed stirring rate in step S2 was changed to 900 r / min, and the ultrasonic treatment time was 30 min, and the remaining steps remained unchanged.

[0094] Example 11: Compared with Example 1, the organic phosphine source in step S1 was changed to hydroxyphosphonoacetic acid, and the remaining steps remained unchanged.

[0095] Example 12: Compared with Example 1, the hydrothermal reaction temperature in step S2 was changed to 180 °C, and the reaction time was 8 h, and the remaining steps remained unchanged.

[0096] Example 13: Compared with Example 1, the low-temperature calcination temperature in step S2 was changed to 500 °C and the time was 5 h; the high-temperature calcination temperature was 700 °C and the time was 7 h, and the remaining steps remained unchanged.

[0097] 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, and ultrasonic treatment was not performed, and the remaining steps remained unchanged.

[0098] Comparative Example 3: Compared with Example 1, the plasma device was not used for treatment during the water bath heating in step S1, and the polymer additive was not added in step S2, and the remaining steps remained unchanged.

[0099] 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; the stirring rate in step S2 was changed to 200 r / min, ultrasonic treatment was not performed, and the polymer additive was not added, and the remaining steps remained unchanged.

[0100] Test Example 1 The performance of the lithium iron phosphate cathode materials of Examples 1-13 and Comparative Examples 1-4 was tested, and the results are shown in Tables 5 and 6.

[0101] Table 5 Detection results of the lithium iron phosphate cathode materials of Examples 1-8 and Comparative Example 1

[0102] According to Tables 1 and 5, it can be seen that the lithium iron phosphate cathode materials have the same chemical composition and corresponding different S 2 (D), Da, D

[200] and D

[200] / D a , so crystal parameter factors Q of different sizes are obtained, and then the lithium iron phosphate cathode materials exhibit different electrochemical performances. Specifically as follows: (1) It can be seen from Examples 1-8 that S 2(D), the smaller the Da value, the larger the D

[200] / Da value, and the larger the obtained crystal parameter factor Q value. At this time, the lithium iron phosphate cathode material exhibits better low-temperature electrochemical performance and high-rate charge and discharge performance. The reason is that in the present invention, by adjusting the above parameters, the lithium iron phosphate cathode material crystal obtains a relatively 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 not only help to improve the specific surface area and tap density of the lithium iron phosphate cathode material, but also help the lithium iron phosphate cathode material particles to maintain a smooth lithium ion transmission channel during charge and discharge in extreme environments, shorten the diffusion path of lithium ions, which greatly improves the deintercalation / insertion efficiency of lithium ions and the electronic conductivity of the material, thereby reducing the low-temperature charge transfer impedance and the sheet resistivity of the electrode, reducing the polarization phenomenon, and improving the electrochemical performance under extreme conditions.

[0103] (2) As can be seen from Table 1 and Table 5, when the variance value S of the grain size of the crystal characteristic growth plane of the lithium iron phosphate cathode material 2 (D), the average grain size value D of all crystal orientations of the material crystal a and D

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

[200] : 15 - 85 nm; D

[200] / D a : 0.6 - 0.9), and the lithium iron phosphate cathode material simultaneously satisfies that the crystal parameter factor Q is within the range value of 0.03 nm -2 ~ 0.15 nm -2 range, the lithium iron phosphate cathode material has a lower low-temperature charge impedance and sheet resistivity, and relatively excellent electrochemical performance.

[0104] (3) According to Table 5, the S 2 (D), D

[200] and Da values of Example 2 are within the specified parameter range, and the electrochemical performance of the corresponding lithium phosphate cathode material is improved compared with that of the lithium phosphate cathode material of Comparative Example 1. Among them, the S 2 (D), D

[200] , D a of Comparative Example 1 are not within the parameter range specified in the examples of the present invention.

[0105] (4) According to Table 1 and Table 5, the S 2 (D), D

[200] , Da and D

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

[0106] (5) According to Table 1 and Table 5, it can be seen that for S in Examples 1 and 4 - 8 2 (D), D

[200] , D a and D

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

[0107] (6) According to Table 1 and Table 5, it can be seen that by comparing Examples 1 - 8 with Comparative Example 1, for S in Comparative Example 1 2 (D), D

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

[200] / Da value meets the specified range, the electrochemical performance of the obtained cathode material in extreme environments is still poor, and the internal resistance of the battery is relatively high.

[0108] It can be seen that when S 2 (D), D

[200] , D a and D

[200] / D a the four variables meet the specified value range in the embodiments of the present invention, and the crystal parameter factor is also within the preferred range, the performance indicators of the obtained lithium iron phosphate cathode material are the most excellent. At least, it is also necessary to satisfy S 2 (D), D

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

[0109] Table 6 Detection results of lithium iron phosphate cathode materials in Examples 9 - 13 and Comparative Examples 2 - 4

[0110] (1) From the comprehensive data in Table 2 and Table 6, it can be seen that from the data of Example 1 and Comparative Examples 3 - 4, when the plasma jet is not used and the polymer additive is not added during the preparation process, the variance value S 2 (D) of the crystal characteristic growth plane grain size of the materials in Comparative Examples 3 and 4 is relatively large, indicating that the crystal growth rate in each direction is uneven and shows anisotropy, the grain shape is irregular, and the obtained crystal parameter factor Q value is also relatively 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 internal resistance of the lithium iron phosphate cathode material battery is relatively high, and both the low-temperature electrical performance and the high-rate electrical performance are relatively poor.

[0111] (2) Compared with Example 1, the plasma treatment time in Example 9 is longer, and the concentration and dosage of the polymer additive are higher. This is conducive to promoting the primary nucleation of lithium iron phosphate and regulating its crystal structure, thereby obtaining grains with regular morphology, increasing the contact area between particles, improving the migration efficiency of lithium ions and the electronic conductivity of the material, and enhancing the extreme electrical properties of the cathode material.

[0112] (3) As can be seen from Table 2 and Table 6, from the data of Example 1, Comparative Example 2 and Comparative Example 4, when no organic phosphonic acid is used and high-speed stirring and ultrasonic treatment are not carried out during the preparation process, the average grain size value D of all crystal orientations of the materials in Comparative Example 2 and Comparative Example 4 a is larger, and the value of D

[200] / D a is smaller, indicating that the diffusion path of lithium ions inside the material crystal is longer, and the exposure degree of the low-surface-energy

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

[0113] (4) Compared with Example 1, Example 10 increases the high-speed stirring rate and extends the ultrasonic treatment time, which is conducive to obtaining lithium iron phosphate crystals with small grain sizes and material particles with a relatively large specific surface area, further promoting the diffusion of lithium ions and the improvement of the electronic conductivity of the material; while Example 11 uses hydroxyphosphonoacetic acid as the organic phosphonic acid. Compared with the single chelating group (amino group) in Example 1, the chelating groups of this organic phosphorus source are both hydroxyl and carboxyl. Compounds with this structure are more likely to adsorb on the surface of primary lithium iron phosphate particles to regulate their crystal structure, reduce the surface energy of the

[200] crystal plane, slow down its growth rate, increase the exposure of this crystal plane in the crystal, and thus reduce the local lattice defects generated during charge and discharge of the material in extreme environments, promote the conduction of lithium ions, and improve the electrochemical performance of the cathode material.

[0114] (5) Compared with Example 1, the extreme environment electrochemical performance of the lithium iron phosphate cathode materials obtained in Example 12 by changing the hydrothermal reaction temperature and in Example 13 by changing the calcination temperature is similar to that of Example 1. This is related to the fact that the three have similar crystal parameter factors in terms of numerical values. Moreover, within a certain temperature range, a higher calcination temperature is conducive to forming a stable, continuous and dense carbon coating layer structure, which can improve the electronic conductivity of the material.

[0115] Detection Example 2 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. The results are shown in Figures 1 to 3 .

[0116] Among them, Figure 1 is the XRD pattern of the lithium iron phosphate cathode material prepared in Example 1 and Comparative Example 2, Figure 2 is the SEM image of the lithium iron phosphate cathode material prepared in Example 1, Figure 3 is the SEM image of the lithium iron phosphate cathode material prepared in Comparative Example 4.

[0117] According to the Figure 1 XRD pattern, it can be seen that the diffraction curves of Example 1 and Comparative Example 2 correspond to the standard card of the lithium iron phosphate cathode material. Compared with Example 1, the diffraction peak intensity of the lithium iron phosphate cathode material in Comparative Example 2 at the

[200] crystal plane is weaker, indicating that this crystal plane is less exposed in the material crystal. Figures 2 - 3 The SEM image of

[0118] shows that the particles of the lithium iron phosphate cathode material prepared in Example 1 are regular in shape and evenly distributed, while the particles of the lithium iron phosphate cathode 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. The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope 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 7 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 positive electrode 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 1wt%-5wt%.

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 heating treatment and the polymer additive solution are stirred at high speed and ultrasonically treated, and then a hydrothermal reaction is carried out, and after the reaction is completed, drying is carried out 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.

6. The preparation method according to claim 5, characterized in that: The conditions of the plasma jet 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 nozzle of the plasma jet and the surface of the reaction solution of 5-8 cm; and a plasma jet processing time of the reaction solution of 30-50 min; The heating temperature is 70-80°C.

7. The preparation method according to claim 5 or 6, characterized in that: The reaction solution also 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.

8. 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 the D element; 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).

9. The preparation method according to claim 5 or 8, characterized in that: The high-speed stirring rate is 600-1000r / min; the ultrasonic treatment time is 20-30min; The conditions of the hydrothermal reaction include: a temperature of 150-180°C and a time of 8-10 hours; 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.

10. A lithium ion battery, characterized in that: It includes the lithium iron phosphate positive electrode material as claimed in claim 1.

Images