Lithium manganese iron phosphate positive electrode material with core-shell structure

Abstract

The invention discloses a lithium manganese iron phosphate positive electrode material with a core-shell structure, which is prepared by the following steps: mixing a manganese source, an iron source, a phosphorus source, lithium carbonate and a carbon source, grinding and drying to prepare a pre-sintered material; then carrying out gas crushing on the pre-sintered material to obtain a pre-gas crushed material, and carrying out heat preservation at the temperature of 200-600 DEG C; putting the pre-gas crushed material subjected to heat preservation into an iron salt solution for quenching reaction to obtain iron compound coated lithium manganese iron phosphate; and finally, mixing the lithium iron manganese phosphate coated with the iron compound, a lithium source, a phosphorus source and a carbon source, grinding, drying, and carrying out secondary calcination to obtain the lithium iron manganese phosphate material with the core-shell structure. During preparation of the lithium manganese iron phosphate positive electrode material, a lithium manganese iron phosphate pre-sintered body and an iron salt solution are subjected to a quenching reaction to form an iron compound coated lithium manganese iron phosphate pre-sintered body, and then the iron compound coated lithium manganese iron phosphate pre-sintered body and other raw materials of lithium iron phosphate are subjected to a secondary calcination reaction to generate the composite positive electrode material with a core-shell structure. A coating structure is formed, the reaction between manganese and electrolyte is reduced, the dissolution of manganese is inhibited, the Gingtaler effect of manganese is relieved, and the stability of the material is improved; and the material has the advantages of high energy density, high power density, excellent cycle performance and strong core-shell interface bonding force.

Description

Technical Field

[0001] The present invention belongs to the field of lithium iron manganese phosphate cathode materials, and particularly relates to a lithium iron manganese phosphate cathode material with a core-shell structure. Background Art

[0002] Lithium-ion batteries utilize the reversible Faraday reaction in the bulk phase to achieve energy storage and conversion, and have the advantages of long cycle life, no memory effect, high specific energy, environmental friendliness, etc., and are widely used in electrochemical energy storage devices. The cathode material of lithium-ion batteries is an important component of lithium-ion batteries and plays a decisive role in the performance of lithium-ion batteries. Olivine-type lithium iron phosphate (LiFePO 4 ) is one of the lithium-ion battery cathode materials that have successfully achieved commercial application. Its advantages such as high safety, high temperature resistance, overcharge resistance, long cycle life, and low cost make it have good prospects for application in power batteries. However, LiFePO 4 is a slightly distorted hexagonal close-packed structure, belonging to the Pmna space group, without a continuous FeO 6 edge-sharing octahedral network, and cannot form effective electronic conduction, resulting in a lithium iron phosphate conductivity of only 10 -9 -10 -10 S / cm, and an ion diffusion coefficient of 1.8×10 -14 cm 2 / s. Therefore, its capacity attenuation is relatively large during high-current discharge, and its rate performance at high current is poor. In addition, the relatively low specific capacity (170 mAh g -1 ) and working voltage (3.45 V vs. Li / Li + ) result in difficulty in further improving the energy density of LiFePO 4 batteries.

[0003] On this basis, by doping an appropriate amount of manganese into the lithium iron phosphate material, lithium iron manganese phosphate (LiMnxFe 1-x PO 4 ) with high discharge platform and good low-temperature working performance emerged. LiMnxFe 1-x PO 4 has a similar olivine structure to LiFePO 4 , and its working voltage reaches 4.10 V, and the energy density is about 20% higher than that of LiFePO 4 . Although the energy density of lithium iron manganese phosphate has been improved, due to its relatively low electronic conduction (10 -13 S / cm) and ionic conductivity (about 10 - 13S / cm, which is 3-4 orders of magnitude lower than that of lithium iron phosphate, making it difficult to fully utilize its capacity. In addition, manganese dissolves during cycling, leading to poor capacity retention, and the Jahn-Teller effect occurs, causing crystal deformation, resulting in relatively low power density and poor electrochemical cycling stability of lithium manganese iron phosphate batteries, unable to complete high-power rapid charging and discharging, thus limiting their practical applications.

[0004] To address the existing problems, a lithium manganese iron phosphate cathode material with a core-shell structure is now developed, which has excellent high energy density, high power density, cycling performance, and strong core-shell interface bonding force. Summary of the Invention

[0005] Object of the Invention: The technical problem to be solved by the present invention is to provide a composite cathode material with lithium manganese iron phosphate as the core layer and lithium iron phosphate as the shell layer, which not only has excellent high energy density, high power density, cycling performance, but also has a strong core-shell interface bonding force.

[0006] Technical Solution: The core-shell structured lithium manganese iron phosphate cathode material of the present invention includes the following steps:

[0007] (1) Mix a manganese source, an iron source, a phosphorus source, a lithium source, and a carbon source, grind and dry them, and keep them reacting at 450-780 °C for 4-15 h to obtain a pre-sintered lithium manganese iron phosphate material, and the carbon content in the pre-sintered lithium manganese iron phosphate material is less than 0.5%;

[0008] (2) Crush the pre-sintered material by air to obtain a pre-air-crushed material, and keep it reacting at 200-600 °C for 0.5-2 h;

[0009] (3) Put the pre-air-crushed material after heat preservation into an iron salt solution for quenching reaction. After stirring and reacting for 0.1-3 h, filter and separate the solid and liquid to obtain lithium manganese iron phosphate coated with an iron compound;

[0010] (4) Mix the lithium manganese iron phosphate coated with an iron compound, a lithium source, a phosphorus source, and a carbon source, grind and dry them, and then perform secondary calcination at 600-850 °C for 8-28 h to obtain the core-shell structured lithium manganese iron phosphate cathode material.

[0011] The present invention is based on the structure of a composite cathode material with lithium iron manganese phosphate as the core layer and lithium iron phosphate as the shell layer. During preparation, one of the raw materials of lithium iron manganese phosphate, namely iron salt, is used as the iron salt solution. The pre-gas-crushed lithium iron manganese phosphate after heat treatment is subjected to a quenching reaction with the iron salt solution to form a pre-sintered material of lithium iron manganese phosphate coated with an iron compound. Then, the pre-sintered material of lithium iron manganese phosphate coated with the iron compound is mixed with a phosphorus source, a lithium source, and a carbon source for secondary calcination to form the composite cathode material. By means of the common raw material iron salt of lithium iron manganese phosphate and lithium iron phosphate, on the one hand, the iron salt decomposes during quenching to form an iron compound coating on the surface of the pre-sintered body of lithium iron manganese phosphate. And during quenching and secondary calcination, thermal diffusion can occur between lithium iron manganese phosphate and lithium iron phosphate with similar phase structures, the two-phase structures are compatible, and a transition layer of two-phase fusion is formed at the interface, improving the bonding ability between the prepared core layer and shell layer. Finally, not only a composite cathode material with lithium iron manganese phosphate as the core layer and lithium iron phosphate as the shell layer is formed, but also the bonding force between the two layers is strong. And during the secondary sintering process, the lithium iron manganese phosphate crystals melt and grow, further improving the compaction performance of the prepared composite cathode material.

[0012] Meanwhile, the present invention strictly controls the residual carbon content in the pre-sintered material, that is, the carbon content in the pre-sintered material of lithium iron manganese phosphate is limited to less than 0.5% to avoid carbon coating under the premise of fully reducing to obtain a pure-phase pre-sintered material of lithium iron manganese phosphate, so that the iron coating layer formed by the subsequent quenching decomposition reaction can be in direct contact with LMFP, so that the elements on the surface of LMFP and the iron coating layer can undergo thermal diffusion with each other during the second calcination process, the two-phase structures are compatible, and a transition layer of two-phase fusion is formed at the interface to improve the bonding force, as Figure 2 shown.

[0013] In addition, the iron coating layer plays a catalytic role during the secondary sintering process, which will increase the graphitization degree of the carbon coating layer, thereby improving the problem of insufficient conductivity of lithium iron manganese phosphate.

[0014] Furthermore, in step (1) of the preparation of the cathode material of the present invention, the molar ratio of the manganese source, iron source, phosphorus source, and lithium source is x:(1 - x):y:z; where 0.45 ≤ x ≤ 1, 1 ≤ y ≤ 1.05, 1.01 ≤ z ≤ 1.10.

[0015] Furthermore, in step (1) of the preparation of the cathode material of the present invention, the addition amount of the carbon source is 6 - 12% of the mass of the theoretically generated lithium iron manganese phosphate.

[0016] Furthermore, in step (2) of the preparation of the cathode material of the present invention, the particle size range of the pre-gas-crushed material is: D10 > 0.1um, D50: 0.3 - 2um, D100 < 10um.

[0017] Furthermore, in the preparation step (3) of the positive electrode material of the present invention, the mass ratio of the iron salt to the pre-air crushed material is (0.001-0.1):1; the concentration of the iron salt solution is 0.01-5 mol / L.

[0018] Furthermore, in the preparation step (3) of the positive electrode material of the present invention, the iron salt is one or more of ferrous acetate, ferrous bromide, ferrous chloride, ferrous sulfate, ferrous nitrate, ferric bromide, ferric chloride, ferric sulfate, ferric perchlorate, ferric nitrate, ferric tribromide, ferric perchlorate, iron dihydrogen phosphate, ferric acetate, ferric formate, ferric citrate, ferric lactate, ferric tartrate or ferric malate.

[0019] Furthermore, in the preparation step (4) of the positive electrode material of the present invention, the molar ratio of the lithium source to lithium and iron in the iron salt in step (3) is (1.01-1.10):1.

[0020] Furthermore, in the preparation step (4) of the positive electrode material of the present invention, the molar ratio of the phosphorus source to phosphorus and iron in the iron salt in step (3) is (1-1.05):1.

[0021] Furthermore, in the preparation step (4) of the positive electrode material of the present invention, the addition amount of the carbon source is 5-15% of the mass of the theoretically generated lithium iron phosphate manganese in step (1).

[0022] Furthermore, in the preparation steps (1) and (4) of the positive electrode material of the present invention, the manganese source is one or more of manganese carbonate, manganese tetroxide, manganese iron oxide or manganese iron precursor; the iron source is one or more of iron phosphate, iron tetroxide, manganese iron oxide, manganese iron precursor; the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, lithium phosphate, lithium dihydrogen phosphate or ammonium monophosphate; the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium oxide, lithium dihydrogen phosphate, lithium phosphate, lithium sulfate, lithium nitrate or lithium chloride precursor; the carbon source is one or more of glucose, sucrose, starch, cellulose, polyvinyl alcohol, soluble starch, cellulose, ascorbic acid or phenolic resin.

[0023] Beneficial effects: Compared with the prior art, the significant advantages of the present invention are as follows: The lithium iron phosphate manganese positive electrode material forms a lithium iron phosphate manganese pre-sintered body coated with an iron compound by quenching reaction of the lithium iron phosphate manganese pre-sintered body with an iron salt solution during preparation, and then undergoes a secondary calcination reaction with other raw materials of lithium iron phosphate to generate a core-shell structure composite positive electrode material. It not only forms a coating structure, reduces the reaction of manganese with the electrolyte, inhibits the dissolution of manganese, alleviates the Jahn-Teller effect of manganese, and improves the stability of the material; but also has a high energy density, a high power density, excellent cycling performance, and a strong core-shell interface bonding force. Description of the Drawings

[0024] Figure 1Flow chart of the preparation method of the core-shell structured lithium iron manganese phosphate cathode material of the present invention;

[0025] Figure 2 Schematic diagram of the influence of the residual carbon content in the lithium iron manganese phosphate pre-sintered material on the lithium iron manganese phosphate and the lithium iron phosphate coating layer;

[0026] Figure 3 First charge-discharge curve diagram of Example 1 and Comparative Example 1;

[0027] Figure 4 SEM image of the core-shell structured lithium iron manganese phosphate cathode material after cycling in Example 1;

[0028] Figure 5 SEM image of the core-shell structured lithium iron manganese phosphate cathode material after cycling in Comparative Example 4. Specific implementation mode

[0029] The technical solution of the present invention will be further described in detail below in conjunction with examples and drawings.

[0030] Example 1

[0031] The lithium iron manganese phosphate cathode material of this Example 1 has the following technological process Figure 1 as shown and is prepared by the following steps:

[0032] (1) Mix manganese carbonate, iron phosphate, phosphoric acid, and lithium carbonate according to the molar ratio of Mn, Fe, P, and Li of 0.6:0.4:1.03:1.03, and add glucose accounting for 7.5% of the theoretically produced lithium iron manganese phosphate material. After grinding and drying, send it into a high-temperature furnace and keep it at 550°C for 8 hours. After cooling, take it out to obtain a pre-sintered material, and the carbon content in this sintered material is 0.12%;

[0033] (2) Carry out air crushing on the pre-sintered material, control the particle size to be D10>0.1um, D50:0.8um, D100<10um to obtain a pre-air-crushed material;

[0034] (3) Prepare an iron salt solution with a concentration of 1mol / L, that is, add iron acetate to pure water and stir to disperse evenly; the addition amount of iron acetate is 0.08:1 based on the mass ratio of the pre-sintered material;

[0035] (4) Send the pre-air-crushed material into a high-temperature furnace at 250°C and keep it for 2 hours;

[0036] (5) Immediately take out the pre-air-crushed material in the furnace while it is hot and pour it into the iron salt solution. After stirring for 1 hour, filter and separate the solid and liquid. After washing and separating the solid several times, dry it to obtain lithium iron manganese phosphate coated with iron compounds;

[0037] (6) Mix lithium iron phosphate, lithium carbonate, ammonium dihydrogen phosphate, and glucose coated with an iron compound. The molar ratio of Li to Fe in lithium carbonate to iron acetate in step (3) is 1.03:1; the molar ratio of P to Fe in ammonium dihydrogen phosphate to iron acetate in step (3) is 1.03:1; the amount of glucose added is 10% of the mass of the theoretically produced lithium iron manganese phosphate in step (1). After grinding and drying, it is sent into a high-temperature furnace and kept at 720 °C for 10 h, then taken out after cooling to obtain a core-shell structured lithium iron manganese phosphate material, namely, an outer carbon coating layer, a middle lithium iron phosphate layer, and an inner lithium iron manganese phosphate layer.

[0038] Comparative Example 1

[0039] This Comparative Example 1 is basically the same as Example 1, the difference being that only a carbon coating layer is formed, which specifically includes the following steps:

[0040] (1) Mix manganese carbonate, iron phosphate, phosphoric acid, and lithium carbonate in a molar ratio of Mn:Fe:P:Li of 0.6:0.4:1.03:1.03, and add 7.5% glucose of the theoretically produced lithium iron manganese phosphate material. After grinding and drying, it is sent into a high-temperature furnace and kept at 550 °C for 8 h, then taken out after cooling to obtain a pre-sintered material;

[0041] (2) Air crush the pre-sintered material, controlling the particle size to be D10 > 0.1 μm, D50: 0.8 μm, D100 < 10 μm to obtain a pre-air-crushed material;

[0042] (3) Mix the pre-air-crushed material with glucose, and the amount of glucose added is 10% of the mass of the theoretically produced lithium iron manganese phosphate in step (1). After grinding and drying, it is sent into a high-temperature furnace and kept at 720 °C for 10 h, then taken out after cooling to obtain a carbon-coated lithium iron manganese phosphate material.

[0043] Comparative Example 2

[0044] The basic steps are the same as those in Example 1, the difference being that only a carbon-iron-coated lithium iron manganese phosphate material is formed, which specifically includes the following steps:

[0045] (1) Mix manganese carbonate, iron phosphate, phosphoric acid, and lithium carbonate in a molar ratio of Mn:Fe:P:Li of 0.6:0.4:1.03:1.03, and add 7.5% glucose of the theoretically produced lithium iron manganese phosphate material. After grinding and drying, it is sent into a high-temperature furnace and kept at 550 °C for 8 h, then taken out after cooling to obtain a pre-sintered material;

[0046] (2) Air crush the pre-sintered material, controlling the particle size to be D10 > 0.1 μm, D50: 0.8 μm, D100 < 10 μm to obtain a pre-air-crushed material;

[0047] (3) Prepare an iron salt solution with a concentration of 1 mol / L, that is, add iron acetate to pure water and stir to disperse evenly; the addition amount of iron acetate is in a mass ratio of 0.08:1 to the pre-sintered material;

[0048] (4) Feed the pre-crushed material into a high-temperature furnace at 250 °C and keep it warm for 2 h;

[0049] (5) Take out the pre-crushed material in the furnace immediately while it is at high temperature and pour it into the iron salt solution. After stirring for 1 h, filter and separate the solid and liquid. After washing and separating the solid several times, dry it to obtain lithium iron phosphate manganese coated with iron compound;

[0050] (6) Mix the lithium iron phosphate manganese coated with iron compound and glucose. The addition amount of glucose is 10% of the mass of the theoretically generated lithium iron phosphate manganese in step (1); After grinding and drying, feed it into a high-temperature furnace and keep it warm at 720 °C for 10 h. Take it out after cooling to obtain the carbon-iron coated lithium iron phosphate manganese material.

[0051] Comparative Example 3

[0052] The basic steps are the same as those in Example 1, except that the pre-crushing process in step (2) is not carried out first. The specific steps are as follows:

[0053] (1) Mix manganese carbonate, iron phosphate, phosphoric acid, and lithium carbonate according to the molar ratio of Mn, Fe, P, and Li of 0.6:0.4:1.03:1.03, and add 7.5% of glucose of the theoretically generated lithium iron phosphate manganese material. After grinding and drying, feed it into a high-temperature furnace and keep it warm at 550 °C for 8 h. Take it out after cooling to obtain the pre-sintered material;

[0054] (2) Prepare an iron salt solution with a concentration of 1 mol / L, that is, add iron acetate to pure water and stir to disperse evenly; the addition amount of iron acetate is in a mass ratio of 0.08:1 to the pre-sintered material;

[0055] (3) Feed the pre-sintered material into a high-temperature furnace at 250 °C and keep it warm for 2 h;

[0056] (4) Take out the pre-crushed material in the furnace immediately while it is at high temperature and pour it into the iron salt solution. After stirring for 1 h, filter and separate the solid and liquid. After washing and separating the solid several times, dry it to obtain lithium iron phosphate manganese coated with iron compound;

[0057] (5) Mix iron compound-coated lithium iron manganese phosphate, lithium carbonate, ammonium dihydrogen phosphate, and glucose. The molar ratio of lithium in lithium carbonate to Li and Fe in iron acetate in step (3) is 1.03:1; the molar ratio of phosphorus in ammonium dihydrogen phosphate to P and Fe in iron acetate in step (3) is 1.03:1; the amount of glucose added is 10% of the mass of the theoretically produced lithium iron manganese phosphate in step (1). After grinding and drying, it is sent into a high-temperature furnace and kept at 720 °C for 10 h, then taken out after cooling to obtain a core-shell structured lithium iron manganese phosphate material, that is, an outer carbon coating layer, a middle lithium iron phosphate layer, and an inner lithium iron manganese phosphate layer.

[0058] Comparative Example 4

[0059] The basic steps are the same as those in Example 1, except that a direct coating process is used to form the core-shell structure, which specifically includes the following steps:

[0060] (1) Mix manganese carbonate, iron phosphate, phosphoric acid, and lithium carbonate according to the molar ratio of Mn:Fe:P:Li of 0.6:0.4:1.03:1.03, and add 7.5% of glucose based on the theoretically produced lithium iron manganese phosphate material. After grinding and drying, it is sent into a high-temperature furnace and kept at 550 °C for 8 h, then taken out after cooling to obtain a lithium iron manganese phosphate material;

[0061] (2) Air crush the pre-sintered material, control the particle size to be D10 > 0.1 um, D50: 0.8 um, D100 < 10 um to obtain a pre-air-crushed material;

[0062] (3) Weigh iron phosphate, phosphoric acid, and lithium carbonate according to the molar ratio of Fe:P:Li of 1:1.03:1.03, mix them with the pre-air-crushed material obtained in step (2) according to the mass ratio of LMFP:LFP = 95:5, and add 7.5% of glucose based on the mass of the lithium iron manganese phosphate material. After grinding and drying, it is sent into a high-temperature furnace and kept at 700 °C for 8 h, then taken out after cooling to obtain a core-shell structured lithium iron manganese phosphate composite material.

[0063] Example 2

[0064] The lithium iron manganese phosphate cathode material of this Example 2 is prepared by the following steps:

[0065] (1) Mix manganese tetraoxide, iron phosphate, phosphoric acid, and lithium carbonate according to the molar ratio of Mn:Fe:P:Li of 0.8:0.2:1.03:1.03, and add 8% of glucose based on the theoretically produced lithium iron manganese phosphate material. After grinding and drying, it is sent into a high-temperature furnace and kept at 600 °C for 6 h, then taken out after cooling to obtain a pre-sintered material, and the carbon content in this sintered material is 0.18%;

[0066] (2) Air crush the pre-sintered material, control the particle size to be D10 > 0.1 um, D50: 1.5 um, D100 < 10 um to obtain a pre-air-crushed material;

[0067] (3) Prepare an iron salt solution with a concentration of 0.05 mol / L, that is, add iron nitrate to pure water and stir to disperse evenly; the addition amount of iron nitrate is in a mass ratio of 0.05:1 to the mass of the pre-calcined material;

[0068] (4) Feed the pre-crushed material into a high-temperature furnace at 450 °C and keep it warm for 2 h;

[0069] (5) Take out the pre-crushed material in the furnace while it is hot and immediately pour it into the iron salt solution. After stirring for 1 h, filter and separate the solid and liquid. After washing and separating the solid several times, dry it to obtain lithium iron phosphate manganese coated with iron compounds;

[0070] (6) Mix lithium iron phosphate manganese coated with iron compounds, lithium carbonate, ammonium dihydrogen phosphate and glucose. The molar ratio of Li to Fe of lithium carbonate to iron nitrate in step (3) is 1.03:1; the molar ratio of P to Fe of ammonium dihydrogen phosphate to iron nitrate in step (3) is 1.03:1; the addition amount of glucose is 10% of the mass of lithium iron phosphate manganese theoretically generated in step (1); after grinding and drying, feed it into a high-temperature furnace and keep it warm at 720 °C for 10 h. After cooling, take it out to obtain a core-shell structured lithium iron phosphate manganese material, that is, an outer carbon coating layer, a middle lithium iron phosphate layer and an inner lithium iron phosphate manganese layer.

[0071] Performance testing

[0072] Prepare the lithium iron phosphate manganese cathode materials obtained in the above Examples 1-2 and Comparative Examples 1-4 into coin cells by the following method and conduct electrochemical performance evaluation:

[0073] Mix the lithium iron phosphate manganese cathode material, Super P (conductive carbon black) and PVDF in a ratio of 8:1:1 in NMP solution to obtain a mixed slurry. Coating the slurry on a bright aluminum foil by hand coating method, and then drying at 100 °C. After the NMP is completely volatilized, punch the electrode sheet into an electrode sheet with a diameter of 13 mm, and then place the electrode sheet in a vacuum oven and dry it overnight at 105 °C. After weighing the electrode sheet, quickly transfer it to a glove box, using metallic lithium as the counter electrode, Celgard 2400 as the separator, and the electrolyte is 1 mol / L LiPF 6 dissolved in a mixed solvent of EC / DMC / EMC (volume ratio of 1:1:1) to assemble the battery; the electrochemical performance test is carried out by a Neware test system, and the charge-discharge behavior of the coin cell is tested and analyzed by using the constant current-constant voltage charging (CC-CV) and constant current discharging (DC) test methods, and the test voltage range is 2.0-4.35 V.

[0074] Manganese dissolution test: For the sample batteries of Examples 1-2 and Comparative Examples 1-4 of the battery, after being fully charged at a rate of 0.1C and then stored at a high temperature of 60°C in an oven for 14 days, all the batteries were taken out and naturally cooled, and then the positive and negative electrodes of the batteries were disassembled. The negative electrode of the sample was digested with a 0.1mol / L aqua regia solution; the content of manganese ions in each sample was measured by ICP, and the manganese content was the dissolved manganese. Then, the ratio of the mass of the dissolved manganese to the total mass of the active material lithium iron phosphate manganese in the corresponding positive electrode sheet was used as the degree of manganese dissolution.

[0075] Table 1 shows the test data of the materials of Examples 1-2 and Comparative Examples 1-4.

[0076]

[0077] As can be seen from the data in Table 1, compared with the comparative examples, the discharge capacity and cycle performance of the core-shell structured lithium iron phosphate manganese cathode material prepared in Examples 1 and 2 of the present invention have been significantly improved; the manganese dissolution of the core-shell structured lithium iron phosphate manganese cathode material in Examples 1 and 2 is significantly lower than that of the comparative examples. Figure 3 The first charge-discharge curve of Example 1 is shown. Compared with Comparative Example 1 with ordinary carbon coating, the capacity is significantly improved, which is the effect of the catalysis of the iron compound on the carbon layer to improve the conductivity; thus, it can be seen that the core-shell structure design of quenching and secondary sintering with an iron salt solution in the present invention benefits from the graphitization catalysis of the iron compound on the carbon and the firm coating obtained by the unique process, and the structural stability and electron / ion conductivity of the lithium iron phosphate manganese cathode material have been significantly improved, and the occurrence of manganese dissolution in the lithium iron phosphate manganese cathode material has been effectively improved, so that the lithium iron phosphate manganese cathode material has excellent electrochemical performance.

[0078] Comparative Example 1 without the protection of the lithium iron phosphate coating layer has a relatively fast capacity decay after long cycling, and the manganese dissolution test confirms that its structure is severely damaged; in Comparative Example 2, the iron oxide coating layer of the carbon-iron coated lithium iron phosphate manganese material is reduced to iron by the carbon source during the secondary sintering process, and the conductivity is improved, but the battery capacity is extremely poor due to the inability of lithium ions to transfer through the iron layer; Comparative Example 3 without the air crushing step has a relatively poor coating effect, so compared with Examples 1 and 2, the capacity decay is more after long cycling, and its manganese dissolution test has the same trend.

[0079] In Comparative Example 4, lithium iron phosphate was directly coated on the surface of the lithium iron phosphate manganese material after air crushing. Air crushing can disperse the lithium iron phosphate manganese, and subsequent coating of lithium iron phosphate can be evenly coated on the surface of the lithium iron phosphate manganese; however, the binding force between the directly coated lithium iron phosphate layer and the core lithium iron phosphate manganese is not strong and cannot provide long-term and stable protection during cycling. Figure 4 and Figure 5The SEM images after cycling of Example 1 and Comparative Example 4 are respectively shown. Thanks to the quenching-two sintering process, the coating layer has a strong binding force with the core lithium iron phosphate manganese. The sample of Example 1 can resist the erosion of the electrolyte during cycling, and the surface remains intact without obvious traces of peeling. In contrast, the particles in Comparative Example 4 are separated, and a complete coating layer cannot be observed on the surface, resulting in a poor coating effect.

[0080] In addition to the above examples, it should be noted that the molar ratio of the manganese source, iron source, phosphorus source, and lithium source used to prepare the lithium iron phosphate manganese core layer of the present invention can be x:(1 - x):y:z; where 0.45 ≤ x ≤ 1, 1 ≤ y ≤ 1.05, 1.01 ≤ z ≤ 1.10; and in the preparation of the lithium iron phosphate manganese core layer and the lithium iron phosphate layer, the manganese source can be at least one of manganese iron oxides or manganese iron precursors, the iron source can be at least one of ferric oxide, manganese iron oxides, and manganese iron precursors, the phosphorus source can be at least one of ammonium dihydrogen phosphate, lithium phosphate, lithium dihydrogen phosphate, or ammonium monophosphate, and the lithium source can be at least one of lithium hydroxide, lithium oxide, lithium dihydrogen phosphate, lithium phosphate, lithium sulfate, lithium nitrate, or lithium chloride precursors; during the preparation of the lithium iron phosphate manganese core layer, the addition amount of the carbon source can be 6 - 12% of the mass of the theoretically generated lithium iron phosphate manganese.

[0081] In the present invention, an iron oxide protective layer is formed by quenching an iron salt solution. The iron salt can be at least one of ferrous acetate, ferrous bromide, ferrous chloride, ferrous sulfate, ferrous nitrate, ferric bromide, ferric chloride, ferric sulfate, ferric perchlorate, ferric tribromide, ferric perchlorate, iron dihydrogen phosphate, iron formate, iron citrate, iron lactate, iron tartrate, or iron malate; since the quenching decomposition temperatures of different iron salts are different, therefore, the holding temperature of lithium iron phosphate manganese can be adjusted according to the iron salt used.

[0082] In the preparation of the lithium iron phosphate layer and the carbon coating layer of the present invention, the molar ratio of lithium to iron in the lithium source and the iron salt used can be (1.01 - 1.10):1; the molar ratio of phosphorus to iron in the phosphorus source and the iron salt used is (1 - 1.05):1; the addition amount of the carbon source can be 5 - 15% of the mass of the theoretically generated lithium iron phosphate manganese.

[0083] That is, by adopting the preparation process of the present invention and the defined parameter ranges, the technical effects claimed by the present invention can be achieved, and thus no further separate examples will be given for verification.

Claims

1. A core-shell structured lithium manganese iron phosphate positive electrode material, characterized in that: Prepared by the following steps: (1) mixing a manganese source, an iron source, a phosphorus source, a lithium source and a carbon source, grinding and drying, and reacting at 450-780° C. for 4-15 hours to obtain a lithium iron manganese phosphate pre-sintered material, wherein the carbon content of the lithium iron manganese phosphate pre-sintered material is less than 0.5%; (2) gas-crushing the pre-sintered material to obtain pre-gas-crushed material, and heat-retaining the material at a temperature of 200-600° C. for a reaction of 0.5-2 h; (3) placing the pre-aerated crushed material after heat preservation into an iron salt solution for quenching reaction, stirring the reaction for 0.1-3 hours, filtering and separating the solid and liquid to obtain lithium manganese iron phosphate coated with iron compound; (4) The iron compound-coated lithium manganese iron phosphate, a lithium source, a phosphorus source and a carbon source are mixed, ground and dried, and then secondary calcined at 600-850° C. for 8-28 h to obtain a core-shell structured lithium manganese iron phosphate positive electrode material.

2. The core-shell structured lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step (1), the molar ratio of the manganese source, iron source, phosphorus source and lithium source is x:(1-x):y:z; wherein 0.45≤x≤1, 1≤y≤1.05, 1.01≤z≤1.

10.

3. The core-shell structured lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step (1), the amount of the carbon source added is 6-12% of the mass of the theoretically generated lithium manganese iron phosphate.

4. The core-shell structured lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step (2), the particle size range of the pre-aerated crushed material is: D10>0.1um, D50: 0.3-2um, D100<10um.

5. The core-shell structured lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step (3), the mass ratio of the iron salt to the pre-aerated crushed material is (0.001-0.1):1; the concentration of the iron salt solution is 0.01-5 mol / L.

6. The core-shell structured lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step (3), the iron salt is one or more of ferrous acetate, ferrous bromide, ferrous chloride, ferrous sulfate, ferrous nitrate, ferric bromide, ferric chloride, ferric sulfate, ferric perchlorate, ferric nitrate, ferric tribromide, ferric perchlorate, ferric dihydrogen phosphate, ferric acetate, ferric formate, ferric citrate, ferric lactate, ferric tartrate or ferric malate.

7. The core-shell structured lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step (4), the molar ratio of the lithium source to the lithium and iron in the iron salt of step (3) is (1.01-1.10):

1.

8. The core-shell structured lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step (4), the molar ratio of the phosphorus source to the phosphorus and iron in the iron salt of step (3) is (1-1.05):

1.

9. The core-shell structured lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step (4), the amount of the carbon source added is 5-15% of the mass of the lithium manganese iron phosphate theoretically generated in step (1).

10. The core-shell structured lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The manganese source is one or more of manganese carbonate, manganese trimanganese tetroxide, ferromanganese oxide or ferromanganese precursor; the iron source is one or more of ferric phosphate, ferromanganese tetroxide, ferromanganese oxide or ferromanganese precursor; the phosphorus source is one or more of phosphoric acid, diammonium phosphate, lithium phosphate, lithium dihydrogen phosphate or monoammonium phosphate; the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium oxide, lithium dihydrogen phosphate, lithium phosphate, lithium sulfate, lithium nitrate or lithium chloride precursor; the carbon source is one or more of glucose, sucrose, starch, cellulose, polyvinyl alcohol, soluble starch, cellulose, ascorbic acid or phenolic resin.

Images