Core-shell iron manganese phosphate precursor, modified lithium iron manganese phosphate material, preparation method and application of modified lithium iron manganese phosphate material and lithium ion battery

Abstract

The invention belongs to the field of positive electrode materials, and particularly relates to a core-shell ferromanganese phosphate precursor, a modified lithium ferric manganese phosphate material, a preparation method and application of the modified lithium ferric manganese phosphate material and a lithium ion battery. The core-shell ferromanganese phosphate precursor comprises a core and a shell wrapping the core, and the core is Fe3 (PO4) 2; the shell is MnxFe1-x (PO4) y (ferromanganese phosphate), x is more than or equal to 0.1 and less than or equal to 0.9, and y is more than or equal to 0.67 and less than or equal to 0.75. The invention provides a core-shell ferromanganese phosphate precursor which takes Fe3 (PO4) 2 as a core and MnxFe1-x (PO4) y as a shell, and when the core-shell ferromanganese phosphate precursor is used for preparing lithium ferromanganese phosphate, the problem of component segregation is solved, and the Jahn-Teller effect of manganese is reduced; especially for subsequent doping and coating, doping segregation is expected to be reduced, coating interface impedance is reduced, and rapid charging of the prepared material and cycling stability under large current can be remarkably enhanced.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material preparation technology, specifically to the field of lithium manganese iron phosphate cathode materials. Background Technology

[0002] Lithium manganese iron phosphate (LiMn) x Fe 1-x PO4 (LMFP) is an olivine-type lithium-ion battery cathode material. It combines the high safety and high charge / discharge voltage platform of both LiFePO4 and LiMnPO4 cathode materials and is widely regarded as the next generation of low-cost, high-energy-density lithium-ion battery cathode material.

[0003] With the continuous evolution of technology, existing technologies have also provided some improvement schemes such as doping and coating. The existing methods for preparing lithium manganese iron phosphate cathode materials are mainly high-temperature solid-state and liquid-phase methods. The solid-state method involves mixing raw materials such as lithium, manganese, iron, and phosphorus sources through ball milling or other means, followed by high-temperature sintering to obtain the cathode material. However, its disadvantages include difficulty in achieving atomic-level mixing of elements such as manganese and iron, and the tendency for sintered products to exhibit large particle sizes and poor electrochemical performance. While the liquid-phase method achieves atomic-level mixing of raw materials more easily than the high-temperature solid-state method, the presence of precipitants (usually ammonia) or flocculants in the precursor preparation process makes it difficult to address the problem of residual ammonium sulfate (or other salts such as sodium sulfate, ammonium chloride, etc.) and organic matter in the final product wastewater.

[0004] For example, patent document CN120854541A discloses a method for preparing lithium manganese iron phosphate cathode material, in which an iron source is deposited in situ on the surface of the lithium manganese iron phosphate core as an anchoring shell; then, lithium iron phosphate is generated in situ using the anchoring shell as an anchor point and coated on the surface of the lithium manganese iron phosphate core as a coating shell. As another example, patent document CN120793882A discloses a fast ion conductor-stabilizer composite coated lithium manganese iron phosphate material, its preparation method, and its application. This lithium manganese iron phosphate material includes a lithium manganese iron phosphate core and a coating layer on its surface; the coating layer is obtained by mixing sodium aluminosilicate, magnesium aluminate, a carbon source, and a lithium supplement agent, coating them on the surface of the lithium manganese iron phosphate core, and then sintering the mixture. Chinese patent document CN120440860A discloses a method for preparing a precursor of lithium manganese iron phosphate. The method involves co-precipitating a mixed metal salt solution and an ammonia solution by concurrently introducing them into the bottom liquid of a reaction vessel under a protective gas atmosphere. The pH of the co-precipitation reaction is 4.5–6.0. ​​The bottom liquid includes a first phosphorus source and ammonium ions, with an ammonium ion concentration of 1.0–2.5 mol / L. The mixed metal salt solution includes a manganese source, an iron source, and a second phosphorus source. The total molar amount of the first and second phosphorus sources is 1.5–2.5 times the total molar amount of manganese and iron. The reaction slurry is then subjected to solid-liquid separation, washing, and drying to obtain ammonium manganese iron phosphate. The ammonium manganese iron phosphate is calcined to obtain manganese pyrophosphate and iron phosphate at a temperature of 400–700 °C.

[0005] In summary, although there are many doping and improvement ideas in the existing technology, the existing technologies are all difficult to solve the problems of metal phase separation and precipitation in the preparation of lithium manganese iron phosphate materials, as well as the interface problems between the coating layers. Therefore, it is difficult for them to meet the requirements of fast charging and long-term cycle stability under high current. Summary of the Invention

[0006] To address the problems of segregation, suboptimal interface structure, and unsatisfactory performance under fast charging and high current in existing lithium manganese iron phosphate materials, the primary objective of this invention is to provide a special core-shell lithium manganese iron phosphate precursor and its preparation method. This invention aims to solve problems such as doping segregation and uneven coating interface by controlling the precursor, thereby improving the fast charging and high current cycling stability of the material.

[0007] The second objective of this invention is to provide a modified lithium iron manganese phosphate material, its preparation, and its application.

[0008] A third objective of this invention is to provide a lithium-ion battery comprising the modified lithium iron manganese phosphate material.

[0009] Unlike lithium iron phosphate, lithium manganese iron phosphate is prone to segregation due to the lattice doping of manganese (Mn), especially with other metal dopants, which further increases the risk of segregation. Furthermore, the interface impedance between the core and coating structure is high, and the introduction of manganese also introduces the inherent Jahn-Teller effect; all of these factors can negatively impact its fast charging and high-current cycling stability. To address these issues, this invention provides the following improvement:

[0010] A core-shell manganese iron phosphate precursor includes a core and a shell encapsulating the core, wherein the core is Fe3(PO4)2 and the shell is Mn. x Fe 1-x (PO4) y (Also known as manganese iron phosphate), where 0.1≤x≤0.9, 0.67≤y≤0.75.

[0011] This invention provides a method using Fe3(PO4)2 as the core and Mn as the core. x Fe 1-x (PO4) y Using core-shell iron manganese phosphate precursors as the shell for the preparation of lithium manganese iron phosphate helps solve the problem of separation and reduce the Jahn-Teller effect of manganese. In particular, for subsequent doping and coating, it is expected to reduce doping separation and reduce the coating interface impedance, which will help to significantly enhance the fast charging and high current cycling stability of the prepared materials.

[0012] In this invention, the Fe / Mn molar ratio in the core-shell manganese iron phosphate precursor can be 1:0.1~9; more specifically, it can be 1:0.2~5.

[0013] The present invention also provides a method for preparing the core-shell manganese iron phosphate precursor, wherein elemental iron and phosphoric acid solution are subjected to a first-stage gas evolution reaction to obtain a first-stage slurry containing dispersed ferrous phosphate solid particles; wherein the final pH of the first-stage gas evolution reaction is controlled at 1.8~3.

[0014] Manganese carbonate is added to a first-stage slurry to carry out a second-stage gas evolution reaction, thereby obtaining the core-shell manganese iron phosphate precursor.

[0015] This invention pre-prepares a slurry containing both solid and dissolved ferrous phosphate through a first-stage gas evolution reaction. A second-stage gas evolution reaction is then performed, allowing the solid ferrous phosphate in the first-stage slurry to act as nucleation centers, forming manganese iron phosphate on its surface. This two-stage gas evolution preparation method helps solve the problem of doping segregation and improves interface stability and reduces interface impedance. The precursor obtained by the method described in this invention is beneficial for preparing lithium manganese iron phosphate materials with fast charging and high-current cycling stability.

[0016] In this invention, the temperature of the first gas evolution reaction is 50~100℃, and more preferably 80~90℃; the Fe / P molar ratio is 1:(1.2~10.0), and more preferably 1:2~7;

[0017] In a slurry, the iron content of the ferrous phosphate solid particles is 20-80% of the total iron content, and can be further 30-60%.

[0018] In this invention, the Fe / Mn molar ratio of iron in a slurry to Mn in manganese carbonate is 1:0.1~9; more specifically, it can be 1:0.2~5.

[0019] The temperature of the second stage gas evolution reaction is 50~100℃, and can be further increased to 80~90℃;

[0020] The second stage of gas evolution reaction takes 1 to 12 hours, for example, 1 to 3 hours.

[0021] This invention also provides a method for preparing modified lithium iron manganese phosphate material, wherein a core-shell iron manganese phosphate precursor, an aluminum source, a phosphorus source, a lithium source and a carbon source are mixed and subjected to a first-stage calcination treatment to obtain a first-stage calcination material; subsequently, it is mixed with an aluminum compound and a lithium compound and subjected to a second-stage calcination treatment to obtain the modified lithium iron manganese phosphate material.

[0022] The core-shell manganese iron phosphate precursor is the core-shell manganese iron phosphate precursor of this invention.

[0023] In this invention, thanks to the special precursor and the combined treatment of aluminum doping and lithium aluminate, a synergistic effect can be achieved, which can synergistically reduce the doping segregation problem. In addition, it is also beneficial to synergistically induce the tight bonding between the β-LiAlO2 coating layer and the substrate material, and reduce the coating interface impedance. This can improve the fast charging and high current cycling stability of the prepared modified lithium iron manganese phosphate material.

[0024] In this invention, the aluminum source and aluminum compound are individually at least one of alumina, aluminum hydroxide, boehmite, and aluminum phosphate;

[0025] The lithium source and lithium compound are individually at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium phosphate;

[0026] The carbon source includes at least one of glucose, sucrose, citric acid, polyethylene glycol, carbon nanotubes, and vapor-grown carbon fibers;

[0027] The phosphorus source includes at least one of ammonium hydrogen phosphate, ammonium dihydrogen phosphate, and lithium phosphate.

[0028] The total molar amount of iron and manganese in the core-shell manganese iron phosphate precursor and the molar ratio of aluminum source (calculated as Al element), phosphorus source and lithium source are 1:0.001~0.05:0.1~0.5:0.6~1.2; it can be further 1:0.01~0.05:0.25~0.35:0.9~1.1;

[0029] The final carbon source coating amount is 5-15% of the weight of the core-shell manganese iron phosphate precursor, and can be further 10-13.5%;

[0030] The total molar amount of iron and manganese in the core-shell manganese iron phosphate precursor and the molar ratio of lithium and aluminum in the lithium and aluminum compounds are 1:0.05~0.2:0.05~0.2; further, it can be 1:0.08~0.15:0.08~0.15.

[0031] In this invention, the atmosphere for the first and second stages of roasting is a protective atmosphere.

[0032] The temperature for the first stage of roasting is 600~850℃; it can be further increased to 650~750℃.

[0033] The first stage of roasting takes 1 to 24 hours, and can be further extended to 2 to 4 hours.

[0034] Before the first stage of roasting, there is also a pre-roasting process, with a temperature of 350~450℃ and a holding time of 1~12h, which can be further extended to 1~3h.

[0035] The temperature for the second stage of roasting is 450~850℃, and further to 500~700℃;

[0036] The second roasting time is 1~24h, and can be further extended to 4~15h.

[0037] The present invention also provides a modified lithium iron manganese phosphate material prepared by a preparation method.

[0038] The preparation method described in this invention can endow the prepared material with special physicochemical properties, and the material with special properties prepared by the method can unexpectedly achieve both excellent fast charging performance and high current cycling stability.

[0039] The present invention also provides an application of the modified lithium iron manganese phosphate material prepared by the above preparation method, which is used as a positive electrode active material for the preparation of lithium-ion batteries.

[0040] The present invention also provides a lithium-ion battery, wherein the positive electrode active material of the lithium-ion battery includes the modified lithium iron manganese phosphate material prepared by the preparation method described in the present invention.

[0041] Beneficial effects

[0042] This invention provides a method using Fe3(PO4)2 as the core and Mn as the core. x Fe 1-x (PO4) y Using core-shell iron manganese phosphate precursors as the shell for the preparation of lithium manganese iron phosphate helps to solve the problem of separation and reduce the Jahn-Teller effect of manganese; especially for doping and coating, it is expected to reduce doping separation and reduce the coating interface impedance, which will help to significantly enhance the fast charging and high current cycling stability of the prepared materials.

[0043] This invention prepares the special precursor based on a two-stage gas evolution reaction. It innovatively combines this precursor with Al doping and Al coating processes in a synergistic manner, which can synergistically reduce doping segregation, induce interfacial structure, and reduce interfacial impedance. This significantly enhances the fast charging and high-current cycling stability of the prepared material. Attached Figure Description

[0044] Figure 1 The XRD pattern of the manganese iron phosphate precursor prepared in Example 1;

[0045] Figure 2 SEM image of the manganese iron phosphate precursor obtained in Example 1;

[0046] Figure 3 The image shows the SEM image of the lithium iron phosphate cathode material finally prepared in step 3 of Example 1.

[0047] Figure 4 The rate performance diagram of the button battery assembled with the lithium manganese iron phosphate cathode material finally obtained in step 3 of Example 1 is shown.

[0048] Figure 5 The graph shows the cycle performance of a button battery assembled from the lithium iron phosphate cathode material finally prepared in step 3 of Example 1 after 500 cycles at 10C.

[0049] Figure 6 The XRD pattern of the manganese iron phosphate precursor prepared in Comparative Example 1;

[0050] Figure 7 SEM image of the manganese iron phosphate precursor prepared in Comparative Example 1;

[0051] Figure 8 This is a SEM image of the lithium iron phosphate cathode material finally prepared in Comparative Example 1. Detailed Implementation

[0052] Example 1

[0053] Step 1: Preparation of core-shell precursors

[0054] S1-1. Disperse 0.0733 mol of phosphoric acid in 50 mL of deionized water, weigh 0.0366 mol of reduced iron powder and add it to the phosphoric acid solution, and react at 90℃ for 3.5 h to obtain the first slurry. The pH of the first slurry is 2.0 (final pH). At this time, the Fe content in the solid phase component (ferrous phosphate solid) of the slurry accounts for about 50% of the total Fe content of the slurry.

[0055] S1-2. Weigh 0.0568 mol of manganese carbonate (Fe / Mn molar ratio in the slurry is 1:1.55) and add it to the first slurry. React at 90℃ for 1 h to obtain the second slurry.

[0056] S1-3. After the second slurry was naturally cooled and aged for 1 hour, a third slurry was obtained with a pH of 5.36. The third slurry was washed three times with deionized water, and the volume of the slurry after adding deionized water should not be less than 100 mL. After filtration, it was freeze-dried to obtain the core-shell structured manganese iron phosphate precursor. XRD and SEM results are shown in [Figure 1-3]. Figure 1 and Figure 2 .

[0057] Step 2: Al doping:

[0058] S2-1. Using the manganese iron phosphate precursor obtained in step 1 as raw material, 3.5g of manganese iron phosphate precursor (Mn:Fe=6:4) and lithium hydroxide, lithium dihydrogen phosphate, alumina, and anhydrous glucose (the total molar amount of manganese iron in the precursor and the molar ratio of lithium hydroxide, lithium dihydrogen phosphate, and alumina are 1:0.6967:0.3033:0.01, respectively, and glucose is 13.2% of the precursor weight) are added to a 50mL zirconium oxide ball mill jar with a ball-to-material ratio of 13:1. Acetone is used as the solvent and the liquid-to-solid ratio is 2:1. The mixture is ball-milled at 650r / min for 5h to obtain a precursor slurry.

[0059] S2-2. Dry the precursor slurry in a 60℃ forced-air drying oven for 1 hour, then transfer it to a 60℃ vacuum drying oven for 12 hours to ensure that the acetone is completely removed; grind the dried powder evenly with a mortar and pestle to obtain the precursor powder.

[0060] S2-3. The precursor powder was placed in a corundum ceramic boat and placed in a tube furnace. Argon was used as the inert atmosphere. The temperature was increased to 350℃ (T1) at a heating rate of 5℃ / min and calcined for 1h. Then the temperature was increased to 650℃ and calcined for 2h. The furnace was cooled to room temperature to obtain aluminum-doped lithium manganese iron phosphate cathode material.

[0061] Step 3: Al coating:

[0062] S3-1. Using the aluminum-doped lithium manganese iron phosphate cathode material obtained in step 2 as raw material, 3.9 g of the aluminum-doped lithium manganese iron phosphate cathode material, lithium hydroxide, and aluminum oxide (the total molar amount of Fe and Mn in the precursor and the molar ratio of lithium hydroxide to aluminum oxide are 1:0.1:0.05) were added to a 50 mL zirconium oxide ball mill jar with a ball-to-material ratio of 17:1. Acetone was used as the solvent and the liquid-to-solid ratio was 3.5:1. The mixture was ball-milled at 650 r / min for 5 h to obtain a mixed slurry.

[0063] S3-2. The mixed slurry was dried in a 60℃ forced-air drying oven for 1 hour, and then transferred to a 60℃ vacuum drying oven for 12 hours. The dried powder was then ground evenly using a mortar and pestle to obtain a powder in which lithium hydroxide, aluminum oxide and aluminum-doped lithium manganese iron phosphate cathode material were uniformly dispersed.

[0064] S3-3. The powder from S2 was placed in a corundum ceramic boat and then placed in a tube furnace. Using argon as an inert atmosphere, the temperature was increased to 650℃ at a heating rate of 5℃ / min and calcined for 5 hours. After furnace cooling, aluminum-doped lithium iron phosphate cathode material coated with lithium aluminate was obtained (SEM image shown). Figure 3 Of which, the Al doping amount (referring to Al / (Fe+Mn)×100%) is 2 mol%, and the LiAlO2 coating amount (referring to LiAlO2 / (Fe+Mn)×100%) is 10 mol%.

[0065] Electrochemical tests are shown below Figure 4 and Figure 5 .

[0066] Example 2

[0067] Compared with Example 1, the only difference is that in step S1-1, the amount of phosphoric acid added is 0.0977 mol, the reaction temperature is 80℃, the time is 1.5h, and the pH of the first slurry is 1.82; in step S1-2, the amount of manganese carbonate added is 0.1514 mol (the Fe / Mn molar ratio in the slurry is 1:4.13); in step S1-3, the pH of the third slurry is 5.51, and other operations and parameters are the same as in Example 1.

[0068] Example 3

[0069] Compared with Example 1, the difference is that in step S1-1, the amount of phosphoric acid added is 0.0224 mol, the reaction time is 6 h, and the pH of the first slurry is 2.88; in step S1-2, the amount of manganese carbonate added is 0.0108 mol (the Fe / Mn molar ratio in the slurry is 1:0.29); in step S1-3, the pH of the third slurry is 5.11, and other operations and parameters are the same as in Example 1.

[0070] Example 4

[0071] Compared with Example 1, the only difference is that in step S2-1, the Al element in the alumina is 1% of the total molar amount of Fe+Mn in the precursor; the carbon source is citric acid, and its amount is 10% of the precursor weight. Other operations and parameters are the same as in Example 1, and the final doping amount is 1 mol.

[0072] Example 5

[0073] Compared with Example 1, the only difference is that in step S2-1, the Al element in the alumina is 5% of the total molar amount of Fe+Mn in the precursor; in step S2-3, the temperature T1 is 400℃ and the time is 2h; the temperature T2 is 750℃ and the time is 3h. All other operations and parameters are the same as in Example 1, and the final doping amount is 5 mol.

[0074] Example 6

[0075] Compared with Example 1, the only difference is that in step S3-1, the total molar amount of Fe and Mn in the precursor and the molar ratio of lithium hydroxide and aluminum oxide are 1:0.05:0.025. All other operations and parameters are the same as in Example 1, and the final coating amount is 5 mol.

[0076] Example 7

[0077] Compared with Example 1, the only difference is that in step S3-1, the total molar amount of Fe and Mn in the precursor and the molar ratio of lithium hydroxide and aluminum oxide are 1:0.15:0.075; in step S3-3, the calcination temperature is 550℃ and the calcination time is 12h; other operations and parameters are the same as in Example 1; and the final coating amount is 15 mol.

[0078] Comparative Example 1

[0079] Compared to Example 1, the only difference is that in step S1-1, the amount of phosphoric acid is adjusted to make the pH of the first slurry 1.48; all other operations and parameters are the same as in Example 1. The XRD and SEM results of the precursor obtained in step 1 are shown below. Figure 6 and Figure 7 The final SEM image of the cathode material is shown in [image missing]. Figure 8 .

[0080] Comparative Example 2

[0081] Compared with Example 1, the only difference is that in step S1-1, the gas evolution reaction is not used, that is, ferrous sulfate is used to replace the reduced iron in an equal molar amount. All other operations and parameters are the same as in Example 1.

[0082] Comparative Example 3

[0083] Compared to Example 1, the only difference is that in step S1-1, after the reaction is complete, a solid-liquid separation process is performed to filter out the insoluble ferrous phosphate particles. Furthermore, the Fe / Mn ratio and other operations and parameters in step S1-2 are the same as in Example 1. This case did not yield a core-shell structured precursor.

[0084] Comparative Example 4

[0085] Compared with Example 1, the only difference is that in steps S1-2, the two-stage gas evolution reaction described above is not used. That is, manganese sulfate is used instead of manganese carbonate, while the molar amount of Mn and other operations and parameters are the same as in Example 1.

[0086] Comparative Example 5

[0087] Compared with Example 1, the only difference is that the raw materials of steps S1-1 and S1-2 are mixed together and a reaction is carried out. All other operations and parameters are the same as in Example 1.

[0088] Comparative Example 6

[0089] Compared with Example 1, the only difference is that in step S2-1, alumina was not added, and step 3 was not performed. All other operations and parameters are the same as in Example 1.

[0090] Comparative Example 7

[0091] Compared with Example 1, the only difference is that step 3 is not performed; all other operations and parameters are the same as in Example 1.

[0092] Comparative Example 8

[0093] Compared with Example 1, the only difference is that in step S2-1, alumina was not added, while the other operations and parameters are the same as in Example 1.

[0094] Comparative Example 9

[0095] Compared with Example 1, the only difference is that in step S2-1, nickel hydroxide, which is equimolar in amount of aluminum, is used to replace aluminum oxide. All other operations and parameters are the same as in Example 1.

[0096] Comparative Example 10

[0097] Compared with Example 1, the difference is that in step S1-1, the amount of phosphoric acid added is changed to 0.1515 mol, the pH of the first slurry is 0.96, and steps 2 and 3 are not performed. All other operations and parameters are the same as in Example 1.

[0098] test:

[0099] A slurry was prepared by mixing positive electrode material, conductive carbon, and PVDF (dissolved in NMP to form a 5 wt% solution) in a ratio of 8:1:1. This slurry was coated onto Al foil to a thickness of 120 μm. After drying, the coated material was cut into 12 mm positive electrode sheets. Button half-cells were assembled using lithium foil as the negative electrode, Celgard 2500 as the separator, and LB-878 electrolyte for testing. The test temperature was 28℃, and the test voltage range was 2.0-4.5V. Under these conditions, the discharge specific capacity at 1C and 10C, and the capacity retention after 500 cycles at 10C, were tested.

[0100] The physicochemical properties of the manganese iron phosphate precursors prepared in Examples 1-3, Comparative Examples 1-5, and Comparative Example 10 were compared, and the experimental results are shown in Table 1.

[0101]

[0102] The lithium manganese iron phosphate cathode materials prepared in Examples 1-7 and Comparative Examples 1-9 were assembled into button batteries under the above conditions and tested. The discharge specific capacity at 1.0C and 10C and the capacity retention rate after 500 cycles at 10C were compared. The test results are shown in Table 2.

[0103]

[0104] Based on the results in Table 1, comparing Examples 1-3 and Comparative Example 1, it can be found that controlling the pH at the end of step 1 within the range described in the claims can ensure the formation of the lamellar core-shell structure. Meanwhile, as shown in Comparative Examples 2-5 and Comparative Example 10, if the raw materials are changed, causing the pH at the end of step 1 to be outside the range described in the claims, or if no precipitate is found in the slurry used between steps 1 and 2, or if the reactants undergo a simultaneous reaction, the lamellar core-shell precursor cannot be obtained.

[0105] According to the results in Table 2, Examples 1-7 and Comparative Examples 6-8 all used sheet-like core-shell structure precursors, and the final capacity retention rate after 500 cycles at 10C was greater than 99%. Comparative Example 6 did not undergo aluminum doping and lithium aluminate coating, Comparative Example 7 did not undergo lithium aluminate coating, and Comparative Example 8 did not undergo aluminum doping, resulting in a specific capacity at 1.0C and 10C that was much lower than that of Example 1. Although Comparative Example 9 used a sheet-like core-shell structure precursor, the doping element was not aluminum, which is found in lithium aluminate, resulting in a specific capacity and cycle stability that were not as good as those of Example 1.

[0106] In summary, the embodiments of the present invention provide a sheet-like core-shell structured manganese iron phosphate precursor and its preparation method. This preparation method prepares a sheet-like manganese iron phosphate precursor by adding iron and manganese sources to the phosphorus source solution in steps, while simultaneously adjusting the pH of the first slurry and controlling the precipitate content in the first slurry. This constructs a core-shell structure with a highly stable Fe3(PO4)2 core and a high-manganese-content MFP coating, avoiding the problems of uneven mixing of raw materials and component segregation in solid-phase methods. At the same time, sulfate and ammonium ions can be optionally avoided during the preparation process, and the problems of ammonium sulfate and organic matter residues that may occur in traditional liquid-phase methods and sol-gel methods can also be avoided. This invention also provides a method for modifying lithium manganese iron phosphate cathode material. The method uses the above-mentioned manganese iron phosphate precursor as raw material and performs secondary modification through aluminum doping and lithium aluminate coating. The resulting lithium manganese iron phosphate cathode material has the characteristics of high stability and excellent electrochemical performance. Aluminum doping improves the Jahn-Teller effect generated by manganese ions during charging and discharging, while lithium aluminate coating improves the ion migration rate of the cathode material and reduces the interfacial reaction with the electrolyte.

[0107] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A core-shell manganese iron phosphate precursor, comprising a core and a shell enclosing the core, characterized in that, The core is Fe3(PO4)2; the shell is Mn. x Fe 1-x (PO4) y , where 0.1≤x≤0.9, 0.67≤y≤0.

75.

2. A method for preparing the core-shell manganese iron phosphate precursor according to claim 1, characterized in that, A first-stage gas evolution reaction was carried out between elemental iron and phosphoric acid solution to obtain a first-stage slurry containing dispersed ferrous phosphate solid particles; wherein the final pH of the first-stage gas evolution reaction was controlled at 1.8~3. Manganese carbonate is added to a first-stage slurry to carry out a second-stage gas evolution reaction, thereby obtaining the core-shell manganese iron phosphate precursor.

3. The method for preparing the core-shell manganese iron phosphate precursor as described in claim 2, characterized in that, The temperature of the first stage gas evolution reaction is 50~90℃; the Fe / P molar ratio is 1:(1.2~10.0); In a slurry, the iron content of the ferrous phosphate solid particles is 20-80% of the total iron content.

4. The method for preparing the core-shell manganese iron phosphate precursor as described in claim 2, characterized in that, The Fe / Mn molar ratio of iron in a certain slurry to Mn in manganese carbonate is 1:0.1~9; The temperature for the second stage of gas evolution reaction is 50~90℃; The second stage of gas evolution reaction takes 1 to 12 hours.

5. A method for preparing a modified lithium iron manganese phosphate material, characterized in that, The core-shell iron manganese phosphate precursor, aluminum source, phosphorus source, lithium source and carbon source are mixed and subjected to a first-stage roasting treatment to obtain a first-stage roasting material; then it is mixed with aluminum compound and lithium compound and subjected to a second-stage roasting treatment to obtain the modified lithium iron manganese phosphate material. The core-shell manganese iron phosphate precursor is the core-shell manganese iron phosphate precursor of claim 1, and / or the core-shell manganese iron phosphate precursor prepared by any one of the preparation methods of claims 2 to 4.

6. The method for preparing the modified lithium iron manganese phosphate material as described in claim 5, characterized in that, The aluminum source and aluminum compound are at least one of alumina, aluminum hydroxide, boehmite, and aluminum phosphate. The lithium source and lithium compound are individually at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium phosphate; The carbon source includes at least one of glucose, sucrose, citric acid, polyethylene glycol, carbon nanotubes, and vapor-grown carbon fibers; The phosphorus source includes at least one of ammonium hydrogen phosphate, ammonium dihydrogen phosphate, and lithium phosphate. The total molar amount of iron and manganese in the core-shell manganese iron phosphate precursor and the molar ratio of aluminum source, phosphorus source and lithium source are 1:0.001~0.05:0.1~0.5:0.6~1.2; The final coating weight of the carbon source is 5-15% of the weight of the core-shell manganese iron phosphate precursor. The total molar amount of iron and manganese in the core-shell manganese iron phosphate precursor and the molar ratio of lithium and aluminum in the lithium and aluminum compounds are 1:0.05~0.2:0.05~0.

2.

7. The method for preparing the modified lithium iron manganese phosphate material according to any one of claims 5 to 6, characterized in that, The atmosphere for the first and second roasting stages is a protective atmosphere. The temperature for the first stage of roasting is 600~850℃; The first stage of roasting takes 1 to 24 hours; Before the first stage of roasting, there is also a pre-roasting process, with a temperature of 350~450℃ and a holding time of 1~12h; The temperature for the second stage of roasting is 450~850℃; The second roasting time is 1 to 24 hours.

8. A modified lithium iron manganese phosphate material prepared by the preparation method of any one of claims 5 to 7.

9. The application of a modified lithium iron manganese phosphate material prepared by any one of claims 5 to 7, characterized in that, It is used as a cathode material in the preparation of lithium-ion batteries.

10. A lithium-ion battery, characterized in that, The positive electrode active material of the positive electrode of the lithium-ion battery includes the modified lithium iron manganese phosphate material prepared by any one of the preparation methods of claims 5 to 7.

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