Modified Lithium Iron Manganese Phosphate Cathode Material and Its Preparation Method and Application

Abstract

The present invention provides a modified lithium iron manganese phosphate cathode material, a preparation method and an application thereof. The modified lithium iron manganese phosphate cathode material comprises a lithium iron manganese phosphate core and an outer layer located on the surface of the lithium iron manganese phosphate core. The outer layer sequentially comprises a first Li3InO3 layer, a lithium iron manganese phosphate layer and a second Li3InO3 layer in the direction from the core to the outside. The provided modified lithium iron manganese phosphate cathode material improves the electron and ion transport rates at the interface and in the bulk phase, and significantly improves the rate performance and cycle performance of lithium iron manganese phosphate.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and particularly relates to the modification of lithium iron manganese phosphate cathode materials. Background Art

[0002] Lithium iron manganese phosphate (LMFP) batteries are indeed an advanced version of lithium iron phosphate (LFP) batteries. Compared with lithium iron phosphate, lithium iron manganese phosphate materials contain an additional manganese element, which is a metal element with very poor conductivity. Therefore, the reversible deintercalation and intercalation of lithium ions in its structure will not be greatly affected, so lithium iron manganese phosphate still has excellent safety performance. In addition, the addition of manganese can increase the working voltage and improve the energy density of lithium iron phosphate. However, the poor conductivity of lithium iron manganese phosphate materials themselves seriously affects their practical applications. Usually, the particles of lithium iron manganese phosphate are very small, and the processing technology is difficult. In addition, it is an insulator and has poor electrical conductivity. Therefore, it is necessary to explore a multiple optimization modification scheme that combines structural optimization and composite modification to improve the conductivity and structural properties of lithium iron manganese phosphate to the greatest extent. Summary of the Invention

[0003] Aiming at the above technical problems, the purpose of the present invention is to provide a modified lithium iron manganese phosphate cathode material, its preparation method and application.

[0004] To achieve the above purpose, the present invention proposes the following solutions:

[0005] In the first aspect, a modified lithium iron manganese phosphate cathode material is provided, including a lithium iron manganese phosphate core and an outer layer located on the surface of the lithium iron manganese phosphate core. The outer layer sequentially includes a first Li3InO3 layer, a lithium iron manganese phosphate layer, and a second Li3InO3 layer from the core outward.

[0006] Further, the Li3InO3 is a porous fast ion conductor.

[0007] Further, the molar ratio of lithium iron manganese phosphate in the lithium iron manganese phosphate core to Li3InO3 in the first Li3InO3 layer is 1:0.02 - 0.06.

[0008] Further, the molar ratio of lithium iron manganese phosphate in the lithium iron manganese phosphate core to lithium iron manganese phosphate in the lithium iron manganese phosphate layer is 1:1 - 2.

[0009] Further, the molar ratio of lithium iron manganese phosphate in the lithium iron manganese phosphate core to Li3InO3 in the second Li3InO3 layer is 1:0.02 - 0.06.

[0010] In the second aspect, a preparation method of the modified lithium iron manganese phosphate cathode material is provided, including:

[0011] S1. Add an organic amine source to a phosphoric acid solution until the pH value of the solution is 8-9 to obtain Solution A, and use a mixed metal salt solution of iron salt and manganese salt as Solution B; slowly add Solution B to Solution A, and after the first stirring reaction, obtain a first slurry. The first slurry is subjected to solid-liquid separation, washing, and pulping to obtain an iron-manganese phosphate slurry; first add an indium salt solution to the iron-manganese phosphate slurry, then add an organic ligand, and add ammonia water to adjust the pH value of the solution to 7-8. After the second stirring reaction, solid-liquid separation, washing, and drying, obtain Intermediate Product 1;

[0012] S2. Uniformly disperse Intermediate Product 1 in water to obtain a slurry. First add Solution A to the slurry, and then slowly add Solution B to the slurry. After the third stirring reaction, obtain a second slurry. The second slurry is subjected to solid-liquid separation, washing, and pulping to obtain Intermediate Product 2 slurry; continue to first add an indium salt solution to Intermediate Product 2 slurry, then add an organic ligand, and add ammonia water to adjust the pH value of the solution to 7-8. After the fourth stirring reaction, after the reaction is completed, perform solid-liquid separation, washing, and drying to obtain a precursor;

[0013] S3. Mix the precursor material with a lithium source and perform high-temperature lithium sintering to obtain the modified lithium iron manganese phosphate cathode material.

[0014] In a third aspect, a lithium-ion battery is provided, including the aforementioned modified lithium iron manganese phosphate cathode material.

[0015] Compared with the prior art, one or more of the above technical solutions can achieve at least one of the following beneficial effects:

[0016] The provided modified lithium iron manganese phosphate cathode material includes a lithium iron manganese phosphate core and a porous fast ion conductor layer, a lithium iron manganese phosphate layer, and a porous fast ion conductor layer sequentially provided on the surface of the lithium iron manganese phosphate core; in the modified lithium iron manganese phosphate material with this structure, the multi-layer designed porous fast ion conductor layer improves the electron and ion transport speeds at the interface and in the bulk phase, and significantly improves the rate performance and cycling performance of lithium iron manganese phosphate.

[0017] The provided preparation method can prepare a lithium iron manganese phosphate cathode material with a structural design including a lithium iron manganese phosphate core and a porous fast ion conductor layer, a lithium iron manganese phosphate layer, and a porous fast ion conductor layer sequentially provided on the surface of the lithium iron manganese phosphate core. This preparation process is simple, easy to operate, and easy to realize large-scale industrialization. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without creative efforts.

[0019] Figure 1 HRTEM image of the modified lithium iron manganese phosphate cathode material prepared in Example 1.

[0020] Figure 2 High-magnification TEM image of the modified lithium iron manganese phosphate cathode material prepared in Example 1.

[0021] Figure 3 Cycling performance graph of the battery assembled with the cathode materials prepared in Examples 1-3 and Comparative Examples 1-2.

[0022] Figure 4 Rate performance graph of the battery assembled with the cathode materials prepared in Example 1 and Comparative Example 2. Specific embodiments

[0023] Some embodiments provide a modified lithium iron manganese phosphate cathode material, including a lithium iron manganese phosphate core and an outer layer located on the surface of the lithium iron manganese phosphate core. The outer layer sequentially includes a first Li3InO3 layer, a lithium iron manganese phosphate layer, and a second Li3InO3 layer in the direction from the core to the outside.

[0024] In some preferred embodiments, the Li3InO3 is a porous fast ion conductor.

[0025] In some preferred embodiments, the molar ratio of lithium iron manganese phosphate in the lithium iron manganese phosphate core to Li3InO3 in the first Li3InO3 layer is 1:0.02-0.06, such as 1:0.02, 1:0.03, 1:0.04, 1:0.05, 1:0.06, etc.

[0026] In some preferred embodiments, the molar ratio of lithium iron manganese phosphate in the lithium iron manganese phosphate core to lithium iron manganese phosphate in the lithium iron manganese phosphate layer is 1:1-2, such as 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2.0, etc.

[0027] In some preferred embodiments, the molar ratio of lithium iron manganese phosphate in the lithium iron manganese phosphate core to Li3InO3 in the second Li3InO3 layer is 1:0.02-0.06, such as 1:0.02, 1:0.03, 1:0.04, 1:0.05, 1:0.06, etc.

[0028] Some embodiments provide a method for preparing a modified lithium iron manganese phosphate cathode material, including:

[0029] S1. Add an organic amine source to the phosphoric acid solution until the pH value of the solution is 8 - 9 to obtain solution A, and use a mixed metal salt solution of iron salt and manganese salt as solution B; slowly add solution B to solution A, and after the first stirring reaction, obtain the first slurry. The first slurry is subjected to solid-liquid separation, washing, and pulping to obtain an iron-manganese phosphate slurry; first add an indium salt solution to the iron-manganese phosphate slurry, and then add an organic ligand. After the second stirring reaction, the obtained product is subjected to solid-liquid separation and washing to obtain intermediate product one.

[0030] S2. Uniformly disperse intermediate product one in water to obtain a slurry. First add solution A to the slurry, and then slowly add solution B to the slurry. After the third stirring reaction, obtain the second slurry. The second slurry is subjected to solid-liquid separation, washing, and pulping to obtain an intermediate product two slurry; continue to first add an indium salt solution to the intermediate product two slurry, and then add an organic ligand. After the fourth stirring reaction, after the reaction ends, perform solid-liquid separation, washing, and drying to obtain a precursor.

[0031] S3. Mix the precursor material with a lithium source and perform high-temperature lithium sintering to obtain the modified lithium iron manganese phosphate cathode material.

[0032] In some embodiments, in steps S1 and S2, among the mixed metal salts, the manganese salt is one or more of manganese sulfate, manganese acetate, and manganese nitrate.

[0033] In some embodiments, in steps S1 and S2, among the mixed metal salts, the iron salt is one or more of ferrous sulfate, ferrous acetate, and ferrous nitrate.

[0034] In some embodiments, in steps S1 and S2, in the mixed metal salts, the molar ratio of manganese to iron is 3:2 - 1:9.

[0035] In some preferred embodiments, in steps S1 and S2, the organic amine source is at least one of 1,6 - hexanediamine, 1,8 - octanediamine, and 1,6 - heptanediamine.

[0036] In some preferred embodiments, in steps S1 and S2, the molar ratio of the mixed metal salt to phosphoric acid is 1:6 - 12, such as 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, etc.

[0037] In some preferred embodiments, in steps S1 and S2, the indium salt is one or more of indium sulfate, indium trichloride, and indium nitrate.

[0038] In some preferred embodiments, in steps S1 and S2, the organic ligand is one or more of terephthalic acid, phthalic acid, and trimellitic acid.

[0039] In some preferred embodiments, in steps S1 and S2, the molar ratio of the indium salt to the organic ligand is 1:5 to 10, such as 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, etc.

[0040] In some preferred embodiments, in step S1, the total molar amount of the manganese salt and the iron salt to the molar amount of the indium salt is 1:0.02 to 0.06, such as 1:0.02, 1:0.03, 1:0.04, 1:0.05, 1:0.06, etc.

[0041] In some embodiments, the duration of the first stirring reaction is 1 to 3 h, such as 1 h, 2 h, 3 h, etc.; the temperature of the first stirring reaction is 40 to 60 °C, such as 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, etc.

[0042] In some embodiments, the duration of the second stirring reaction is 16 to 30 h, such as 16 h, 18 h, 20 h, 22 h, 24 h, 26 h, 28 h, 30 h, etc.; the temperature of the second stirring reaction is 30 to 90 °C, preferably 40 to 90 °C, such as 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, etc.

[0043] In some embodiments, the duration of the third stirring reaction is 1 to 3 h, such as 1 h, 2 h, 3 h, etc.; the temperature of the third stirring reaction is 40 to 60 °C, such as 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, etc.

[0044] In some embodiments, the duration of the fourth stirring reaction is 16 to 30 h, such as 16 h, 18 h, 20 h, 22 h, 24 h, 26 h, 28 h, 30 h, etc.; the temperature of the fourth stirring reaction is 30 to 90 °C, preferably 40 to 90 °C, such as 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, etc.

[0045] In some embodiments, the lithium source is one or more of lithium hydroxide, lithium nitrate, and lithium acetate.

[0046] In some preferred embodiments, the ratio of the total molar amount of manganese and iron in the precursor material to the molar amount of lithium in the lithium source is 1:1.1 to 1.3, such as 1:1.1, 1:1.15, 1:1.2, 1:1.25, 1:1.3, etc.

[0047] In some preferred embodiments, the atmosphere for the high-temperature lithium sintering is a nitrogen or argon atmosphere.

[0048] In some preferred embodiments, the high-temperature lithium sintering is carried out at 800 to 1000 °C, such as 800 °C, 820 °C, 850 °C, 880 °C, 900 °C, 920 °C, 950 °C, 980 °C, 1000 °C, etc.

[0049] In some preferred embodiments, the time of the high-temperature lithiation sintering is 10 to 30 h, such as 10 h, 12 h, 15 h, 18 h, 20 h, 22 h, 25 h, 28 h, 30 h, etc.

[0050] Some embodiments provide a lithium-ion battery, including the aforementioned modified lithium iron manganese phosphate cathode material.

[0051] To facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the accompanying drawings of the specification and preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0052] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0053] Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in the present invention can be obtained through market purchase or can be prepared by existing methods.

[0054] Example 1

[0055] (1) At 50 °C, 105 ml of concentrated phosphoric acid (85 wt.%) was dissolved in 100 ml of water to form solution A, and 0.1 mol of manganese acetate and 0.1 mol of ferrous acetate were dissolved in 100 ml of deionized water to form solution B. A 1,6-hexanediamine solution was added dropwise to solution A to adjust the pH value of solution A to 8 - 9. Subsequently, the metal salt solution B was added dropwise to solution A. After the first stirring reaction for 2 h, a precipitate was obtained. After filtration, alcohol washing, and water washing, a solid was obtained. The solid and water were mixed and slurried to obtain an iron-manganese phosphate slurry; 0.008 mol of indium sulfate solution was continuously added to the iron-manganese phosphate slurry, and then 0.064 mol of terephthalic acid was added. After reacting at 50 °C for 24 h, an In-MOF-coated iron-manganese phosphate precursor material was formed after filtration and washing. Subsequently, the prepared In-MOF-coated iron-manganese phosphate precursor material was dispersed in 200 ml of deionized water to form a slurry.

[0056] (2) At 50 °C, 105 ml of concentrated phosphoric acid was dissolved in 100 ml of water to form solution A, and 0.1 mol of manganese acetate and 0.1 mol of iron acetate were dissolved in 100 ml of deionized water to form solution B. 1,6-Hexanediamine solution was added dropwise to solution A to adjust the pH value of solution A to 8-9. The pH-adjusted solution A was added to the above-mentioned slurry, and then metal salt solution B was added dropwise to the slurry. After stirring and reacting for 2 h for the third time, a precipitate was obtained. After filtration, alcohol washing, and water washing, a solid was obtained. The solid and water were mixed and slurried to obtain an intermediate product slurry; then 0.008 mol of indium sulfate solution and 0.064 mol of terephthalic acid were added in sequence, and the reaction was carried out at 50 °C for 24 h. After the reaction, the prepared material was filtered, washed with water, alcohol washed, and dried to form a precursor material with a structure of iron-manganese phosphate core - In-MOF layer - iron-manganese phosphate layer - In-MOF layer from the inside to the outside.

[0057] (3) The precursor material prepared in step (2) was subjected to high-temperature lithium sintering at 930 °C for 16 h in a nitrogen atmosphere with 0.48 mol of lithium hydroxide to obtain the modified lithium iron manganese phosphate material.

[0058] The HRTEM image of the obtained modified lithium iron manganese phosphate material is as Figure 1 shown. From Figure 1 it can be seen that the obtained modified lithium iron manganese phosphate material successively includes a lithium iron manganese phosphate core, a lithium indate layer, a lithium iron manganese phosphate layer, and a lithium indate layer from the inside to the outside. Due to the influence of the shooting angle and the internal and external phases, some lattices will be distorted, and interface lattice reconstruction will occur at the interface, so the lattice orientation is somewhat different; the TEM image of the obtained modified lithium iron manganese phosphate material is as Figure 2 shown. From Figure 2 it can be seen that the outer layer is a porous structure material, that is, it can be known that the lithium indate layer is a porous structure. Since the TEM image cannot penetrate the entire material, only the surface can be seen. However, combined with the preparation method (after the MOF template method is subjected to lithium sintering, the obtained material is a porous structure), it can be concluded that the inner lithium indate layer located on the lithium iron manganese phosphate core is also a porous structure material. Thus, it can be seen that the obtained material is a multi-layer porous fast ion conductor composite lithium iron manganese phosphate cathode material with an ABAB structure of lithium iron manganese phosphate - porous fast ion conductor Li3InO3 from the inside to the outside.

[0059] Comparative Example 1

[0060] (1) At 50 °C, 105 ml of concentrated phosphoric acid (85 wt.%) was dissolved in 100 ml of water to form solution A, and 0.1 mol of manganese acetate and 0.1 mol of iron acetate were dissolved in 100 ml of deionized water to form solution B. A 1,6-hexanediamine solution was added dropwise to solution A to adjust the pH value of solution A to 8-9. Subsequently, metal salt solution B was added dropwise to solution A. After stirring and reacting for 2 h for the first time, a precipitate was obtained. After filtration, washing with alcohol, and washing with water, a solid was obtained. The solid was mixed with water to form a slurry to obtain an iron-manganese phosphate slurry; 0.008 mol of indium sulfate solution was continuously added, and 0.064 mol of terephthalic acid was added. After hydrothermal reaction at 200 °C for 20 h, an In-MOF-coated iron-manganese phosphate precursor material was formed after filtration, washing, and drying.

[0061] (2) The precursor material prepared in step (1) was subjected to high-temperature lithium sintering at 930 °C for 16 h with 0.25 mol of lithium hydroxide in a nitrogen atmosphere to prepare a lithium iron manganese phosphate cathode material coated with a porous fast ion conductor Li3InO3 fast ion conductor.

[0062] Comparative Example 2

[0063] (1) At 50 °C, 105 ml of concentrated phosphoric acid (85 wt.%) was dissolved in 100 ml of water to form solution A, and 0.1 mol of manganese acetate and 0.1 mol of iron acetate were dissolved in 100 ml of deionized water to form solution B. A 1,6-hexanediamine solution was added dropwise to solution A to adjust the pH value of solution A to 8-9. Subsequently, metal salt solution B was added dropwise to solution A. After stirring and reacting for 2 h for the first time, a precipitate was obtained. After filtration, washing with alcohol, washing with water, and drying, an iron-manganese phosphate precursor material was formed.

[0064] (2) The precursor material prepared in step (1) was subjected to high-temperature lithium sintering at 930 °C for 16 h with 0.25 mol of lithium hydroxide in a nitrogen atmosphere to prepare a lithium iron manganese phosphate cathode material.

[0065] Example 2

[0066] (1) At 50 °C, 105 ml of concentrated phosphoric acid (85 wt.%) was dissolved in 100 ml of water to form solution A, and 0.1 mol of manganese acetate and 0.1 mol of ferrous acetate were dissolved in 100 ml of deionized water to form solution B. 1,8-Octanediamine solution was added dropwise to solution A to adjust the pH value of solution A to 8 - 9. Subsequently, metal salt solution B was added dropwise to solution A. After the first stirring reaction for 2 h, a precipitate was obtained. After filtration, washing with alcohol and water, a solid was obtained. The solid was mixed with water to form a slurry, and a phosphate slurry of iron and manganese was obtained; 0.008 mol of indium sulfate solution was added to the phosphate slurry of iron and manganese, and then 0.036 mol of ammonia water solution was added. After reacting at 30 °C for 5 h, an In(OH)3-coated precursor material of iron and manganese phosphate was formed after filtration and washing.

[0067] (2) Subsequently, the prepared In(OH)3-coated precursor material of iron and manganese phosphate was dispersed in 200 ml of deionized water to form a slurry. At 50 °C, 105 ml of concentrated phosphoric acid (85 wt.%) was dissolved in 100 ml of water to form solution A, and 0.1 mol of manganese acetate and 0.1 mol of ferrous acetate were dissolved in 100 ml of deionized water to form solution B. 1,8-Octanediamine solution was added dropwise to solution A to adjust the pH value of solution A to 8 - 9. The pH-adjusted solution A was added to the above slurry, and then metal salt solution B was added dropwise to the slurry. After the third stirring reaction for 2 h, a precipitate was obtained. After filtration, washing with alcohol and water, a solid was obtained. The solid was mixed with water to form an intermediate product slurry; 0.008 mol of indium sulfate solution and 0.036 mol of ammonia water solution were successively added to the intermediate product slurry. After reacting at 30 °C for 5 h, after the reaction, the prepared material was filtered, washed with water, washed with alcohol and dried to form a precursor material with a structure of iron and manganese phosphate core - In(OH)3 layer - iron and manganese phosphate layer - In(OH)3 layer from the inside to the outside.

[0068] (3) The precursor material prepared in step (2) was subjected to high-temperature lithium sintering at 930 °C for 16 h with 0.48 mol of lithium hydroxide in a nitrogen atmosphere, and a multilayer fast ion conductor composite lithium iron manganese phosphate cathode material with an ABAB structure from the inside to the outside containing the fast ion conductor Li3InO3 could be prepared.

[0069] Example 3

[0070] (1) At 50 °C, 105 ml of concentrated phosphoric acid (85 wt.%) was dissolved in 100 ml of water to form solution A, and 0.1 mol of manganese acetate and 0.1 mol of ferrous acetate were dissolved in 100 ml of deionized water to form solution B. A 1,6-hexanediamine solution was added dropwise to solution A to adjust the pH value of solution A to 8-9. Subsequently, metal salt solution B was added dropwise to solution A. After the first stirring reaction for 2 h, a precipitate was obtained. After filtration, alcohol washing, and water washing, a solid was obtained. The solid was mixed with water to form a slurry of iron and manganese phosphates; 0.008 mol of indium sulfate solution was added to the slurry of iron and manganese phosphates, and then 0.056 mol of terephthalic acid was added. After reacting at 80 °C for 24 h, an In-MOF-coated iron and manganese phosphate precursor material was formed after filtration and washing. Subsequently, the prepared In-MOF-coated iron and manganese phosphate precursor material was dispersed in 200 ml of deionized water to form a slurry.

[0071] (2) At 50 °C, 105 ml of concentrated phosphoric acid was dissolved in 100 ml of water to form solution A, and 0.15 mol of manganese acetate and 0.15 mol of ferrous acetate were dissolved in 100 ml of deionized water to form solution B. A 1,6-hexanediamine solution was added dropwise to solution A to adjust the pH value of solution A to 8-9. The pH-adjusted solution A was added to the above slurry, and then metal salt solution B was added dropwise to the slurry. After the third stirring reaction for 2 h, a precipitate was obtained. After filtration, alcohol washing, and water washing, a solid was obtained. The solid was mixed with water to form an intermediate product slurry; 0.012 mol of indium sulfate solution and 0.084 mol of terephthalic acid were added in sequence. After reacting at 80 °C for 24 h, after the reaction, the prepared material was filtered, washed with water, alcohol washed, and dried to form a precursor material with a structure of iron and manganese phosphate core-In-MOF layer-iron and manganese phosphate layer-In-MOF layer from the inside to the outside.

[0072] (3) The precursor material prepared in step (2) was subjected to high-temperature lithium sintering at 900 °C for 16 h in a nitrogen atmosphere with 0.63 mol of lithium hydroxide to obtain the modified lithium iron manganese phosphate material.

[0073] The materials prepared in Examples 1-3 and Comparative Examples 1-2 were assembled into batteries by the following method:

[0074] The materials prepared in Examples 1 to 3 and Comparative Examples 1 to 2 were used as the positive electrode materials respectively, and were mixed with the conductive agent acetylene black (AB) and the binder polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1. Using N-methylpyrrolidone (NMP) as the solvent, the mixture was placed in a small beaker and stirred at a speed of 800 r / min for 2 h to obtain a slurry. The slurry was coated on the current collector aluminum foil using an automatic coater, laid flat on tempered glass and transferred to a vacuum drying oven at 85 °C for drying for 4 h. After punching into a pole piece with a diameter of 12 mm, it was dried in a vacuum drying oven at 105 °C for 4 h, and placed in a glove box with a water content and oxygen content both lower than 0.1 ppm and filled with argon atmosphere for 4 h to reduce the moisture adsorbed by the pole piece during the transfer process, and then assembled into a CR2032 type button cell in the glove box. The battery used a pure metal lithium sheet with a diameter of 16 mm and a thickness of 0.5 mm as the negative electrode, and a porous polyethylene film with a diameter of 18 mm and a model of Celgard 2300 as the separator.

[0075] After the above battery was assembled, it was aged for 12 h, and then at a voltage of 2 to 4.4 V, after being activated for 3 cycles at a current density of 0.1 C, it was cycled 100 times at a current density of 1 C. The cycle performance diagram is as Figure 3 shown.

[0076] From Figure 3 it can be seen that compared with the batteries assembled with the unmodified lithium iron manganese phosphate material in Comparative Example 2 and the lithium iron manganese phosphate material coated with the porous fast ion conductor Li3InO3 fast ion conductor in Comparative Example 1, the first discharge capacity of the batteries assembled with the ABAB-structured multi-layer hollow fast ion conductor composite lithium iron manganese phosphate positive electrode materials in Examples 1 and 3 increased, and the cycle performance was significantly improved. After analysis, this may be because the multi-layer designed porous fast ion conductor layer improved the electron and ion transport speeds at the interface and in the bulk phase. As the electrochemical reaction time progressed, the polarization phenomenon of the positive electrode material became more and more obvious. Especially during the high-rate cycling process, the multi-layer designed porous fast ion conductor layers in Examples 1 and 3 could improve the ionic conductivity, reduce the ionic diffusion energy barrier, and effectively inhibit the polarization during the high-rate cycling process, significantly improving the cycle performance.

[0077] From Figure 3 it can also be seen that compared with the batteries assembled with the modified positive electrode materials in Examples 1 and 3, the discharge specific capacity of the batteries assembled with the ABAB-structured multi-layer fast ion conductor composite lithium iron manganese phosphate positive electrode material prepared in Example 2 was slightly lower, and the cycle performance was significantly worse. After analysis, this may be because the special porous channels of the fast ion conductors of the modified positive electrode materials prepared in Examples 1 and 3 provided more convenient channels for the electron and ion transport at the interface and in the bulk phase.

[0078] The batteries assembled with the cathode materials prepared in Example 1 and Comparative Example 2 were aged for 12 h, and then the rate performance test was carried out at a voltage of 2~4.4 V. The results are as Figure 4 shown. It can be seen from Figure 4 that the rate performance of the cathode material in Example 1 is significantly better than that of the cathode material in Comparative Example 2.

[0079] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. Modified lithium iron manganese phosphate cathode material, characterized in that, It includes a lithium iron manganese phosphate core and an outer layer located on the surface of the lithium iron manganese phosphate core. The outer layer sequentially includes a first Li3InO3 layer, a lithium iron manganese phosphate layer, and a second Li3InO3 layer in the direction from the core to the outside.

2. The modified lithium iron manganese phosphate cathode material according to claim 1, wherein The Li3InO3 is a porous fast ion conductor; The molar ratio of lithium iron manganese phosphate in the lithium iron manganese phosphate core to Li3InO3 in the first Li3InO3 layer is 1:0.02 to 0.06; The molar ratio of lithium iron manganese phosphate in the lithium iron manganese phosphate core to lithium iron manganese phosphate in the lithium iron manganese phosphate layer is 1:1 to 2; The molar ratio of lithium iron manganese phosphate in the lithium iron manganese phosphate core to Li3InO3 in the second Li3InO3 layer is 1:0.02 to 0.

06.

3. Preparation method of modified lithium iron manganese phosphate cathode material, characterized in that, It includes: S1. Add an organic amine source to a phosphoric acid solution until the pH value of the solution is 8 to 9 to obtain solution A, and use a mixed metal salt solution of an iron salt and a manganese salt as solution B; Slowly add solution B to solution A, perform a first stirring reaction to obtain a first slurry. The first slurry is subjected to solid-liquid separation, washing, and pulping to obtain an iron-manganese phosphate slurry; first add an indium salt solution to the iron-manganese phosphate slurry, and then add an organic ligand. After a second stirring reaction, the obtained product is subjected to solid-liquid separation and washing to obtain intermediate product one; S2. Uniformly disperse intermediate product one in water to obtain a slurry. First add solution A to the slurry, and then slowly add solution B to the slurry. After a third stirring reaction, a second slurry is obtained. The second slurry is subjected to solid-liquid separation, washing, and pulping to obtain an intermediate product two slurry; continue to first add an indium salt solution to the intermediate product two slurry, and then add an organic ligand. After a fourth stirring reaction, after the reaction ends, it is subjected to solid-liquid separation, washing, and drying to obtain a precursor; S3. Mix the precursor material with a lithium source and perform high-temperature lithium sintering to obtain the modified lithium iron manganese phosphate cathode material; In step S1 and step S2, the molar ratio of the mixed metal salt to phosphoric acid is 1:6 to 12; In step S1 and step S2, the organic ligand is one or more of terephthalic acid, phthalic acid, and trimellitic acid; The temperature of the first stirring reaction is 40 to 60 °C; The temperature of the second stirring reaction is 30 to 90 °C; The temperature of the third stirring reaction is 40 to 60 °C; The temperature of the fourth stirring reaction is 30 to 90 °C.

4. The preparation method of the modified lithium iron manganese phosphate cathode material according to claim 3, characterized in that, In step S1 and step S2, in the mixed metal salt, the manganese salt is one or more of manganese sulfate, manganese acetate, and manganese nitrate; In step S1 and step S2, in the mixed metal salt, the iron salt is one or more of ferrous sulfate, ferrous acetate, and ferrous nitrate; In step S1 and step S2, in the mixed metal salt, the molar ratio of manganese to iron is 3:2 to 1:9; In step S1 and step S2, the organic amine source is at least one of 1,6-hexanediamine, 1,8-octanediamine, and 1,6-heptanediamine; 5. The preparation method of the modified lithium iron manganese phosphate cathode material according to claim 3, characterized in that, In step S1 and step S2, the indium salt is one or more of indium sulfate, indium trichloride, and indium nitrate; In step S1 and step S2, the molar ratio of the indium salt to the organic ligand is 1:5 to 10; In step S1, the ratio of the total molar amount of the manganese salt and the iron salt to the molar amount of the indium salt is 1:0.02 to 0.

06.

6. The preparation method of the modified lithium iron manganese phosphate cathode material according to claim 3, characterized in that The duration of the first stirring reaction is 1 to 3 h; The duration of the second stirring reaction is 16 to 30 h.

7. The preparation method of the modified lithium iron manganese phosphate cathode material according to claim 3, characterized in that, The duration of the third stirring reaction is 1 to 3 h; The duration of the fourth stirring reaction is 16 to 30 h.

8. The preparation method of the modified lithium iron manganese phosphate cathode material according to claim 3, characterized in that, The lithium source is one or more of lithium hydroxide, lithium nitrate, and lithium acetate; The ratio of the total molar amount of manganese and iron in the precursor material to the molar amount of lithium in the lithium source is 1:1.1 to 1.

3.

9. The preparation method of the modified lithium iron manganese phosphate cathode material according to claim 3, wherein The atmosphere for the high-temperature lithiumation sintering is a nitrogen or argon atmosphere; The high-temperature lithiumation sintering is carried out at 800 to 1000 °C; The time for the high-temperature lithiumation sintering is 10 to 30 h.

10. A lithium-ion battery, characterized in that, It includes the modified lithium iron manganese phosphate cathode material as described in claim 1 or 2.

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