Lithium manganese iron phosphate positive electrode material and manufacturing method thereof

Abstract

The invention discloses a lithium manganese iron phosphate positive electrode material and a manufacturing method thereof, the structural formula of the positive electrode material is LiaFebMncPO4AdAleTif, in the formula, an element A is one or more of Mg, Nb and Ta, a is more than 0.00 and less than or equal to 1.10, b is more than or equal to 0.01 and less than or equal to 1.00, c is more than or equal to 0.01 and less than or equal to 1.00, d is more than or equal to 0.0001 and less than or equal to 0.02, e is more than or equal to 0.0001 and less than or equal to 0.02, and f is more than or equal to 0.0001 and less than or equal to 0.02. The positive electrode material has the characteristics of high discharge voltage, high capacity and cycle resistance, and is high in safety performance.

Description

Technical Field

[0001] The invention belongs to the field of lithium ion batteries, and in particular relates to a lithium manganese iron phosphate positive electrode material and a manufacturing method thereof. Background Art

[0002] In recent years, with the portability and cordless nature of various electronic devices, the demand for small, lightweight, and high-energy-density non-aqueous electrolyte secondary batteries has increased, and it is hoped that the development of positive electrode materials for non-aqueous electrolyte secondary batteries with better characteristics than before will be carried out. The positive electrode materials of non-aqueous electrolyte secondary batteries are LiCoO2, LiFePO4, LiMn2O4, etc. Among them, lithium iron phosphate batteries are widely used due to their high safety and excellent cycle performance, but the low energy density limits the scope of use of the material. However, introducing a certain amount of Mn into the lithium iron phosphate material can appropriately increase the discharge platform voltage of the material, thereby increasing the energy density of the material.

[0003] However, due to the low electrical conductivity of lithium manganese iron phosphate, how to improve the electrochemical performance of the material has become an important research topic in the industry. Summary of the invention

[0004] The purpose of the present invention is to provide a lithium manganese iron phosphate positive electrode material and a method for manufacturing the same, so that the positive electrode material has the characteristics of high discharge voltage, high capacity and cycle resistance, and has high safety performance.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] A lithium manganese iron phosphate positive electrode material, the structural formula of which is: Li a Fe b Mn c PO4A d Al e Ti f , where element A is one or more of Mg, Nb, and Ta, 0.00<a≤1.10, 0.01≤b≤1.00, 0.01≤c≤1.00, 0.0001≤d≤0.02, 0.0001≤e≤0.02, and 0.0001≤f≤0.04.

[0007] Preferably, in the formula, 1.0≤c / b≤4, ensuring that the contents of manganese and iron are equivalent to improve the electrochemical activity of the lithium battery, and 0.00<b+c≤1.00 ensures the stoichiometric coefficient of lithium manganese iron phosphate.

[0008] Preferably, in the formula, 0.01≤d / e≤0.6, ensuring that the ratio of A element to Al is appropriately doped, and if it exceeds 0.6 or is less than 0.01, the effect of increasing the capacity will not be achieved. And 0.001≤e+d≤0.04, if it exceeds this range, the effect of increasing the discharge voltage is not obvious, and the effect of charge and discharge cycle durability is not ideal.

[0009] Preferably, in the lithium manganese iron phosphate compound, a portion of Al and A elements replace the solid solution of manganese iron atoms in the lithium manganese iron phosphate compound particles. To improve safety, the amount of Al in the form of oxide is less than 20 mol% of the total Al contained in the lithium manganese iron phosphate compound, preferably less than 10 mol%.

[0010] The lithium manganese iron phosphate positive electrode material of the present invention is preferably spherical nanoparticles, and its average particle size (D50, the same below) is preferably 2 to 20 μm, preferably 3 to 15 μm. When the average particle size is less than 2 μm, it affects the coating of the material on the electrode surface. On the contrary, when it exceeds 20 μm, it is difficult to form a smooth electrode layer surface.

[0011] Preferably, the positive electrode material is in the form of a single particle with good dispersibility, which can not only increase the filling density of the material of the electrode layer, but also improve the charge and discharge rate performance.

[0012] The selected element A of the positive electrode material of the particle morphology of the present invention is actually evenly dispersed on the particle surface, indicating that the stoichiometry of the doping element A on the surface of the material, inside the material, and between each particle is basically the same.

[0013] Preferably, the elements A and Ti exist inside the material, so that the electrochemical properties of the material can be improved by adding appropriate amounts of doping elements A and Ti.

[0014] A part of Al and Ti present in the positive electrode material replaces the solid solution of manganese and iron atoms inside the particles. A part of the element A present in the positive electrode material preferably replaces the solid solution of lithium atoms inside the particles.

[0015] Preferably, the compact density of the positive electrode material of the present invention is 2.0 to 2.4 g / cm 3 . Compressive density is less than 2.0g / cm 3 When the positive electrode sheet is formed using a granular positive electrode material, the initial energy density of the positive electrode decreases, and the compact density is greater than 2.4 g / cm 3 When the initial energy density of the positive electrode decreases or the high rate discharge characteristics decrease, the compact density of the particle morphology positive electrode material is preferably 2.15 to 2.3 g / cm 3 .

[0016] The specific surface area of ​​the positive electrode material of the present invention is preferably 10 to 25 m 2 / g. Specific surface area is less than 10m 2 / g, the initial discharge capacity per unit weight decreases, and on the contrary, when it exceeds 25m 2 / g, the initial discharge capacity per unit volume decreases, and it is impossible to obtain a good positive electrode material. The best specific surface area is 15 to 18 m 2 / g.

[0017] The present invention also provides a method for preparing the lithium iron manganese phosphate positive electrode material, comprising the following steps:

[0018] (1) Lithium carbonate powder and a solvent are fully stirred to prepare a suspension, and a phosphoric acid solution is added to the prepared suspension, and the mixture is stirred and mixed.

[0019] (2) Add iron salt, manganese salt, metal A compound, titanium oxide and sugar source into water, then add the mixed solution into the suspension obtained in step (1), stir, and then dry to obtain lithium manganese iron phosphate precursor.

[0020] (3) calcining the lithium iron manganese phosphate precursor powder under an inert atmosphere, and then crushing it.

[0021] (4) The crushed powder is mixed with an aluminum source and then sintered again in an inert atmosphere to obtain a lithium manganese iron phosphate positive electrode material.

[0022] Preferably, the iron salt may be one or more of ferric phosphate, iron red, ferrous oxalate, and pure iron;

[0023] Preferably, the manganese salt may be one or more of manganese carbonate, manganese tetraoxide, manganese phosphate, manganese hydrogen phosphate, and manganese oxalate; in order to increase the compaction density and obtain a larger crystal distance, acid manganese phosphate is preferred.

[0024] Preferably, the compound of metal A may be one or more of sulfate, hydrochloride, nitrate, oxide, hydroxide, carbonate and the like.

[0025] The molar ratio of manganese to iron is 1.0≤n Mn / n Fe ≤4, the molar ratio of element A to titanium is 0.01≤n A / n Ti ≤0.6.

[0026] Preferably, the sugar source includes one or more of glucose, sucrose, lactose, galactose and starch. Preferably, the added amount of the sugar source is 10-15% of the mass of the iron source.

[0027] Preferably, the drying is spray drying. The inlet temperature of the spray dryer is 200-400°C, the outlet temperature is 80-150°C, and the physicochemical frequency is 50-100HZ to obtain the first stage powder, i.e., lithium manganese iron phosphate precursor.

[0028] Preferably, the solvent is polyethylene glycol, polyvinyl pyrrolidone, polyvinyl alcohol (PVA), or cross-linked polyvinyl pyrrolidone (PVPP).

[0029] Preferably, the phosphoric acid solution is added dropwise, and the phosphoric acid solution can be a phosphoric acid-PEG solution.

[0030] Preferably, in step (3), the calcination temperature is 500-720°C and the calcination time is 8-15h.

[0031] Preferably, in step (3), air flow is used for pulverization, and the classification frequency is controlled at 80 to 150 Hz.

[0032] Preferably, in step (4), the sintering temperature is 200-400° C., and the sintering time is 5-10 hours.

[0033] Preferably, in step (4), the aluminum source is selected from aluminum oxide or aluminum hydroxide.

[0034] In the present invention, various methods can be used as a method for adding Ti, Al, and element A to the ferromanganese raw material powder and the lithium raw material powder. That is, a compound containing any one or more of Ti, Mg, and element A is dissolved or dispersed in an aqueous solution, an organic solvent, etc., and then an organic acid or a hydroxyl-containing organic substance having the ability to form a complex is added to obtain a uniform solution or colloidal solution, so that Ti, Mg, and element A are uniformly dispersed in the Li, Fe, and Mn raw materials. After drying, calcination is carried out in a nitrogen protective atmosphere, and the calcined powder is fully mixed with an Al source and then calcined again to obtain a high-performance battery material.

[0035] The present invention can better control the structure of the battery material by controlling the ratio range of manganese to iron elements and the ratio range of element A to Al in lithium manganese iron phosphate, which not only improves the discharge voltage of the battery material, but also improves the charge and discharge cycle performance of the battery, increases the number of charge and discharge times, and has high safety performance. DETAILED DESCRIPTION

[0036] Source of raw materials: lithium carbonate (Tianqi Lithium), tantalum oxide (West Asia Reagents), niobium oxide (Hefei Tianjian Chemicals)

[0037] Instrument model: Spray dryer (Shanghai Dachuanyuan), model (OPD-8)

[0038] X-ray Diffractometer - PANalytical AERIS Desktop

[0039] Example 1

[0040] 188 mg of lithium carbonate powder was fully stirred with 8 ml of PEG solution for 1 hour, and 8 ml of PEG solution containing 344 mg of phosphoric acid was added dropwise to the suspension, and stirring was continued for 30 minutes after the addition was completed. At the same time, 300 mg of iron phosphate, 363 mg of manganese carbonate, 1.65 mg of titanium oxide, 0.35 mg of magnesium sulfate and 39 mg of sucrose were fully mixed in 3 ml of deionized water, and this solution was continuously added to the above lithium carbonate / phosphoric acid / PEG suspension, kept stirring at room temperature for 30 minutes, and then spray dried. The spray dryer inlet temperature was 300 ° C, the outlet temperature was 100 ° C, and the physical and chemical frequency was 60HZ to obtain lithium manganese iron phosphate precursor powder.

[0041] The lithium iron manganese phosphate precursor powder was kept at 680°C for 10 hours in a sintering furnace protected by a nitrogen atmosphere, and then crushed to obtain the second stage powder. The second stage powder was fully mixed with 0.373 mg of mixed phase alumina, and the mixture was sintered in a nitrogen atmosphere at 300°C for 6 hours to obtain the final lithium iron manganese phosphate compound Li 1.02 Fe 0.4 Mn 0.6 PO4Mg 0.0006 Al 0.001 Ti 0.004 .

[0042] The specific surface area of ​​the calcined powder obtained by nitrogen adsorption method was 18.32 m 2 / g. The average particle size D50 obtained by a laser scattering particle size distribution meter is 1.22μm. After observation using SEM (scanning electron microscope), it was found that the obtained positive electrode material powder contained agglomerates formed by about 1 to 10 primary particles, but the proportion of them was low, and most of them were nano-scale small particles. The calcined powder was subjected to a high-sensitivity X-ray diffraction method using Cu-Kα rays at a scanning speed of 1° / min. The results showed that Mg and Ti did not exist in the form of separate oxides, but about 10mol% of Al existed in the form of oxides.

[0043] The Li prepared above was mixed with the mixture in a weight ratio of 90 / 5 / 5. 1.02 Fe 0.4 Mn 0.6 PO4Mg 0.0006 Al 0.001 Ti 0.004 The powder, high-conductive carbon and polytetrafluoroethylene powder are mixed while adding toluene, and dried to obtain a positive electrode plate with a thickness of 150 μm.

[0044] A 20μm thick aluminum foil was used as the positive electrode collector, a 25μm thick porous polypropylene was used as the separator, a 500μm thick metal lithium foil was used as the negative electrode, a 20μm nickel platinum was used as the negative electrode collector, and 1M LiPF6 / EC+DEC (EC and DEC mass ratio 1:1) was used as the electrolyte. A button cell was assembled in an argon glove box.

[0045] First, the battery was charged to 4.3 V at 25° C. with a load current of 75 mA per 1 g of positive electrode active material, and then discharged to 2.0 V with a load current of 75 mA per 1 g of positive electrode active material to obtain an initial discharge capacity. The battery charge and discharge cycle test was performed 15 times.

[0046] The initial discharge capacity at 25°C, 2.0-4.3V, and a discharge rate of 0.1C is 150.5mAh / g, and the average voltage is 4.135V. The capacity retention rate after 15 charge and discharge cycles is 99.3%. The compact density of the positive electrode powder is 2.42g / cm3.

[0047] [Example 2]

[0048] The positive electrode active material was synthesized in the same manner as in Example 1 except that niobium oxide was used instead of magnesium sulfate, and composition analysis, physical property determination and battery performance test were performed. 1.02 Fe 0.4 Mn 0.6 PO4Nb 0.0006 Al 0.001 Ti 0.004 .

[0049] The specific surface area of ​​the calcined powder obtained by nitrogen adsorption method is 22.56m2 / g, and the average particle size D50 obtained by laser scattering particle size distribution meter is 1.12μm. The initial discharge capacity under the conditions of 25℃, 2.75~4.3V, and discharge rate 0.1C is 149.0mAh / g, and the average voltage is 4.035V. The capacity retention rate after 15 charge and discharge cycles is 98.2%. In addition, the compact density of the positive electrode powder is 2.26g / cm 3 .

[0050] In addition, the X-ray diffraction spectrum of the calcined powder was obtained by high-sensitivity X-ray diffraction using Cu-Kα rays. The measurement conditions were a scanning speed of 1° / min and a step angle of 0.02°. As a result, about 8 mol% of aluminum existed in the form of a single oxide, but magnesium and niobium did not exist in the form of oxides.

[0051] [Example 3]

[0052] The positive electrode active material was synthesized in the same manner as in Example 1 except that tantalum oxide was used instead of magnesium sulfate, and composition analysis, physical property determination and battery performance test were performed. 1.02 Fe 0.4 Mn 0.6 PO4Ta 0.0006 Al 0.001 Ti 0.004 .

[0053] The specific surface area of ​​the calcined powder obtained by nitrogen adsorption method was 24.51 m 2 / g, and the average particle size D50 obtained by laser scattering particle size distribution meter is 1.03μm. The initial discharge capacity under 25℃, 2.75~4.3V, and discharge rate 0.1C is 149.3mAh / g, and the average voltage is 3.935V. The capacity retention rate after 15 charge and discharge cycles is 98.7%. In addition, the compact density of the positive electrode powder is 2.16g / cm 3 The X-ray diffraction spectrum of the calcined powder was obtained by high-sensitivity X-ray diffraction using Cu-Kα rays. The measurement conditions were a scanning speed of 1° / min and a step angle of 0.02°. As a result, about 12 mol% of aluminum existed in the form of a single oxide, but magnesium and niobium did not exist in the form of oxides.

[0054] Example 4

[0055] The lithium manganese iron phosphate compound was prepared by the same method as in Example 1, except that the molar ratio of the added manganese element to the iron element was 1:1, and the molar ratio of the added element Mg to Al was 0.05. The structural formula was Li 1.02 Fe 0.5 Mn 0.5 PO4Mg 0.0005 Al 0.01 Ti 0.004 .

[0056] The specific surface area of ​​the calcined powder obtained by nitrogen adsorption method was 23.65 m 2 / g, and the average particle size D50 obtained by laser scattering particle size distribution meter is 1.21μm. The initial discharge capacity under 25℃, 2.75~4.3V, and discharge rate 0.1C conditions is 148.2mAh / g, and the average voltage is 4.0535V. The capacity retention rate after 15 charge and discharge cycles is 98.5%. In addition, the compact density of the positive electrode powder is 2.16g / cm 3The X-ray diffraction spectrum of the calcined powder was obtained by high-sensitivity X-ray diffraction using Cu-Kα rays. The measurement conditions were a scanning speed of 1° / min and a step angle of 0.02°. As a result, about 10 mol% of aluminum existed in the form of a single oxide, but magnesium and niobium did not exist in the form of oxides.

[0057] Example 5

[0058] The lithium manganese iron phosphate compound was prepared by the same method as in Example 1, except that the molar ratio of the added manganese element to the iron element was 7:3, the molar ratio of the added element Mg to Al was 0.02, and the structural formula was Li 1.02 Fe 0.3 Mn 0.7 PO4Mg 0.0002 Al 0.01 Ti 0.004 .

[0059] The specific surface area of ​​the calcined powder obtained by nitrogen adsorption method is 22.83 m 2 / g, and the average particle size D50 obtained by laser scattering particle size distribution meter is 1.17μm. The initial discharge capacity under 25℃, 2.75~4.3V, and discharge rate 0.1C conditions is 132.2mAh / g, and the average voltage is 4.1825V. The capacity retention rate after 15 charge and discharge cycles is 97.7%. In addition, the compact density of the positive electrode powder is 2.46g / cm 3 The X-ray diffraction spectrum of the calcined powder was obtained by high-sensitivity X-ray diffraction using Cu-Kα rays. The measurement conditions were a scanning speed of 1° / min and a step angle of 0.02°. As a result, about 9 mol% of aluminum existed in the form of a single oxide, but magnesium and niobium did not exist in the form of oxides.

[0060] Comparative Example 1

[0061] The positive electrode active material was synthesized in the same manner as in Example 1 except that no alumina powder, magnesium carbonate powder, and titanium oxide powder were used, and composition analysis, physical property measurements, and battery performance tests were performed. 1.02 Fe 0.4 Mn 0.6 PO4.

[0062] The specific surface area of ​​the calcined powder was 18.50 m 2 / g, and the average particle size D50 obtained by laser scattering particle size distribution meter is 1.25μm. The initial discharge capacity under 25℃, 2.75~4.3V, and discharge rate 0.1C is 145.2mAh / g, and the average voltage is 3.964V. The capacity retention rate after 15 charge and discharge cycles is 97.9%. In addition, the compact density of the positive electrode powder is 2.36g / cm 3 The X-ray diffraction spectrum of the calcined powder was obtained by a high-sensitivity X-ray diffraction method using Cu-Kα rays. The measurement conditions were a scanning speed of 1° / min and a step angle of 0.02°, and the result showed lithium manganese iron phosphate.

[0063] Comparative Example 2

[0064] The positive electrode active material was synthesized in the same manner as in Example 1 except that titanium oxide was not used, and the composition analysis, physical property measurement and battery performance test were performed. 1.02 Fe 0.4 Mn 0.6 PO4Mg 0.0006 Al 0.001 .

[0065] The specific surface area of ​​the calcined powder obtained by nitrogen adsorption method was 18.34 m 2 / g, and the average particle size D50 obtained by a laser scattering particle size distribution meter was 1.28μm. Aluminum was present on the surface. In addition, the compact density of the positive electrode powder was 2.30g / cm 3 .

[0066] The initial discharge capacity at 25°C, 2.75-4.3V, and a discharge rate of 0.5C is 148.0 mAh / g, and the average voltage is 3.964 V. The capacity retention rate after 15 charge and discharge cycles is 98.0%.

[0067] [Comparative Example 3]

[0068] The positive electrode active material was synthesized in the same manner as in Example 1 except that magnesium sulfate was not used, and composition analysis, physical property measurements, and battery performance tests were performed. 1.02 Fe 0.4 Mn 0.6 PO 45 Al 0.001 Ti 0.004 .

[0069] The specific surface area of ​​the calcined powder obtained by nitrogen adsorption method was 18.57 m 2 / g, and the average particle size D50 obtained by a laser scattering particle size distribution meter is 1.35μm. Aluminum and titanium exist on the surface. In addition, the compact density of the positive electrode powder is 2.34g / cm3 .

[0070] The initial discharge capacity at 25°C, 2.75-4.3V, and a discharge rate of 0.5C is 144.2mAh / g, and the average voltage is 4.164V. The capacity retention rate after 15 charge and discharge cycles is 98.1%.

[0071] [Comparative Example 4]

[0072] The positive electrode active material was synthesized in the same manner as in Example 1 except that the molar ratio of Mn to Fe was less than 1, i.e., c / b < 1, and the composition analysis, physical property determination, and battery performance test were performed. 1.02 Fe 0.6 Mn 0.4 PO4Mg 0.0006 Al 0.001 Ti 0.004 .

[0073] The specific surface area of ​​the calcined powder obtained by nitrogen adsorption method was 19.20 m 2 / g, and the average particle size D50 obtained by laser scattering particle size distribution meter is 1.28μm. The initial discharge capacity under 25℃, 2.75~4.3V, and discharge rate 0.1C conditions is 143.2mAh / g, and the average voltage is 3.744V. The capacity retention rate after 15 charge and discharge cycles is 98.1%. In addition, the compact density of the positive electrode powder is 2.43g / cm 3 The X-ray diffraction spectrum of the calcined powder was obtained by a high-sensitivity X-ray diffraction method using Cu-Kα rays. The measurement conditions were a scanning speed of 1° / min and a step angle of 0.02°, and the result showed lithium manganese iron phosphate.

[0074] [Comparative Example 5]

[0075] The positive electrode active material was synthesized in the same manner as in Example 1 except that the molar ratio of Mg to Al was greater than 0.5, i.e., c / b was greater than 0.5, and the composition analysis, physical property determination, and battery performance test were performed. 1.02 Fe 0.6 Mn 0.4 PO4Mg 0.003 Al 0.001 Ti 0.004 .

[0076] The specific surface area of ​​the calcined powder obtained by nitrogen adsorption method was 18.35 m 2 / g, and the average particle size D50 obtained by laser scattering particle size distribution meter is 1.15μm. The initial discharge capacity under 25℃, 2.75~4.3V, and discharge rate 0.1C conditions is 139.2mAh / g, and the average voltage is 4.054V. The capacity retention rate after 15 charge and discharge cycles is 98.5%. In addition, the compact density of the positive electrode powder is 2.33g / cm 3 The X-ray diffraction spectrum of the calcined powder was obtained by a high-sensitivity X-ray diffraction method using Cu-Kα rays. The measurement conditions were a scanning speed of 1° / min and a step angle of 0.02°, and the result showed lithium manganese iron phosphate.

[0077] [Comparative Example 6]

[0078] The positive electrode active material was synthesized in the same manner as in Example 1 except that the molar ratio of Mn to Fe was greater than 4, i.e., c / b>4, and the composition analysis, physical property determination, and battery performance test were performed. 1.02 Fe 0.1 Mn 0.9 PO4Mg 0.0006 Al 0.001 Ti 0.004 .

[0079] The specific surface area of ​​the calcined powder obtained by nitrogen adsorption method was 17.20 m 2 / g, and the average particle size D50 obtained by a laser scattering particle size distribution meter is 1.48μm. The initial discharge capacity under the conditions of 25°C, 2.75-4.3V, and a discharge rate of 0.1C is 139.2mAh / g, and the average voltage is 4.134V. The capacity retention rate after 15 charge and discharge cycles is 98.7%. In addition, the compact density of the positive electrode powder is 2.53g / cm3. The X-ray diffraction spectrum of the calcined powder was obtained by using a high-sensitivity X-ray diffraction method using Cu-Kα rays. The measurement conditions are a scanning speed of 1° / min and a step angle of 0.02°, and the results show lithium manganese iron phosphate.

[0080] [Comparative Example 7]

[0081] The positive electrode active material was synthesized in the same manner as in Example 1 except that the molar ratio of Mg to Al was 0.8, i.e., d / e>0.6, and the composition analysis, physical property measurement, and battery performance test were performed. 1.02 Fe 0.6 Mn 0.4 PO4Mg 0.0008 Al 0.001 Ti 0.004 .

[0082] The specific surface area of ​​the calcined powder obtained by nitrogen adsorption method was 18.55 m2 / g, and the average particle size D50 obtained by laser scattering particle size distribution meter is 1.18μm. The initial discharge capacity under 25℃, 2.75~4.3V, and discharge rate 0.1C is 140.2mAh / g, and the average voltage is 4.063V. The capacity retention rate after 15 charge and discharge cycles is 98.1%. In addition, the compact density of the positive electrode powder is

[0083] 2.35g / cm3. The X-ray diffraction spectrum of the calcined powder was obtained by high-sensitivity X-ray diffraction using Cu-Kα rays. The measurement conditions were a scanning speed of 1° / min and a step angle of 0.02°, and the result showed lithium manganese iron phosphate.

[0084] [Comparative Example 8]

[0085] The lithium iron manganese phosphate precursor powder is kept at 680°C in a sintering furnace protected by a nitrogen atmosphere for 10 hours, and then crushed to obtain the second stage powder. The above-mentioned second stage powder is fully mixed with 1.492 mg of mixed phase alumina, and the mixture is sintered in a nitrogen protective atmosphere at a sintering temperature of 300°C for 6 hours to obtain the final lithium iron manganese phosphate compound. The rest is consistent with Example 1 to synthesize the positive electrode active material, and perform composition analysis, physical property measurement and battery performance test. The molar fraction of oxidized Al is increased to 40 mol% by doping, and the structure is Li 1.02 Fe 0.4 Mn 0.6 PO4Mg 0.0005 Al 0.004 Ti 0.004 .

[0086] The specific surface area of ​​the calcined powder obtained by nitrogen adsorption method was 18.47 m 2 / g, and the average particle size D50 obtained by laser scattering particle size distribution meter is 1.17μm. The initial discharge capacity under 25℃, 2.75~4.3V, and discharge rate 0.1C conditions is 138.3mAh / g, and the average voltage is 4.133V. The capacity retention rate after 15 charge and discharge cycles is 97.0%. In addition, the compact density of the positive electrode powder is 2.29g / cm 3 The X-ray diffraction spectrum of the calcined powder was obtained by a high-sensitivity X-ray diffraction method using Cu-Kα rays. The measurement conditions were a scanning speed of 1° / min and a step angle of 0.02°, and the result showed lithium manganese iron phosphate.

Claims

1. A lithium manganese iron phosphate positive electrode material, characterized in that: Its structural formula is: Li a Fe b Mn c PO4A d Al e Ti f , wherein element A is one or more of Mg, Nb, Ta, 0.00<a≤1.10, 0.01≤b≤1.00, 0.01≤c≤1.00, 0.0001≤d≤0.02, 0.0001≤e≤0.02, 0.0001≤f≤0.

02.

2. The positive electrode material according to claim 1, characterized in that In the formula, 1.0≤c / b≤4, and 0.00<b+c≤1.

00.

3. The positive electrode material according to claim 1 or 2, characterized in that In the formula, 0.01≤d / e≤0.6, and 0.001≤e+d≤0.

04.

4. The positive electrode material according to any one of claims 1 to 3, characterized in that The average particle size of the lithium manganese iron phosphate positive electrode material is 2 to 20 μm, preferably 3 to 15 μm; Preferably, the specific surface area of ​​the lithium manganese iron phosphate positive electrode material is 10 to 25 m 2 / g, preferably 15 to 18 m 2 / g.

5. The method for preparing the lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 4, comprising the following steps: (1) stirring lithium carbonate powder and a solvent to uniformly prepare a suspension, adding a phosphoric acid solution to the prepared suspension, and stirring and mixing; (2) adding iron salt, manganese salt, metal A compound, titanium oxide and sugar source into water, then adding the mixed solution into the suspension obtained in step (1), stirring, and then drying to obtain lithium manganese iron phosphate precursor; (3) calcining the lithium iron manganese phosphate precursor powder under the protection of an inert atmosphere, and then crushing it; (4) The crushed powder is mixed with an aluminum source and then sintered again in an inert atmosphere to obtain a lithium manganese iron phosphate positive electrode material.

6. The preparation method according to claim 5, characterized in that: The iron salt is one or more of ferric phosphate, red iron, ferrous oxalate and pure iron; Preferably, the manganese salt is one or more of manganese carbonate, manganese tetraoxide, manganese phosphate, manganese hydrogen phosphate, and manganese oxalate; preferably acid manganese phosphate; Preferably, the compound of metal A is one or more of sulfate, hydrochloride, nitrate, oxide, hydroxide, and carbonate; Preferably, the molar ratio of manganese to iron is 1.0≤n Mn / n Fe ≤4, the molar ratio of element A to titanium is 0.01≤n A / n Ti ≤0.

6.

7. The preparation method according to claim 5 or 6, characterized in that: The sugar source includes one or more of glucose, sucrose, lactose, galactose and starch. Preferably, the added amount of the sugar source is 10-15% of the mass of the iron source.

8. The preparation method according to any one of claims 5 to 7, characterized in that: The drying is spray drying; the inlet temperature of the spray dryer is 200-400°C, the outlet temperature is 80-150°C, and the physicochemical frequency is 50-100HZ, to obtain a stage 1 powder, i.e., a lithium manganese iron phosphate precursor; Preferably, the solvent is polyethylene glycol, polyvinyl pyrrolidone, polyvinyl alcohol, or cross-linked polyvinyl pyrrolidone; Preferably, the phosphoric acid solution is added dropwise.

9. The preparation method according to any one of claims 5 to 8, characterized in that: In step (3), the calcination temperature is 500-720° C. and the calcination time is 8-15 h; Preferably, in step (3), air flow is used for pulverization, and the classification frequency is controlled at 80 to 150 Hz; Preferably, in step (4), the sintering temperature is 200-400° C. and the sintering time is 5-10 h; Preferably, in step (4), the aluminum source is selected from aluminum oxide or aluminum hydroxide.