Methods for preparing ferromanganese oxalate precursor and lithium iron manganese phosphate positive electrode material

Abstract

The present invention relates to a new method for preparing a ferromanganese oxalate precursor. The method comprises the following steps: mixing a water-soluble manganese salt, a water-soluble iron salt, a doping auxiliary agent 1 and a water-soluble oxalate in a water-organic solvent system, and subjecting same to a coprecipitation reaction. The present invention further relates to a method for preparing a lithium iron manganese phosphate positive electrode material. The method comprises the following steps: mixing the ferromanganese oxalate precursor with a lithium source, a phosphorus source and a doping auxiliary agent 2, drying same, then subjecting same to low-temperature roasting to obtain a lithium iron manganese phosphate precursor, mixing the precursor with an organic carbon source, then drying the mixture, and subjecting the mixture to high-temperature sintering, so as to obtain a carbon-coated lithium iron manganese phosphate positive electrode material. The present invention further relates to the ferromanganese oxalate precursor and the lithium iron manganese phosphate positive electrode material prepared by these methods.

Description

Preparation method of manganese iron oxalate precursor and lithium manganese iron phosphate positive electrode material Technical Field

[0001] The invention relates to a preparation method of a manganese iron oxalate precursor and a lithium manganese iron phosphate positive electrode material. Background Art

[0002] Adding a certain amount of manganese to the lithium iron phosphate cathode material creates a higher voltage platform, reaching around 4.1V, compared to around 3.4-3.5V for lithium iron phosphate. While both have the same theoretical specific capacity, the higher voltage gives lithium manganese iron phosphate a 15-20% higher theoretical energy density under the same conditions. Therefore, lithium manganese iron phosphate cathode material has become an upgraded version of lithium iron phosphate and has garnered significant attention.

[0003] The difficulty of lithium iron manganese phosphate positive electrode materials lies mainly in how to synthesize them, and the synthesis methods mainly include liquid phase method and high-temperature solid phase method.

[0004] Chinese patent CN107732176B describes a method for preparing a nanoscale lithium-ion battery positive electrode material, which includes the following steps: preparing a lithium iron manganese phosphate precursor solution; preparing a conductive coating layer precursor solution; mixing the lithium iron manganese phosphate precursor solution and the conductive coating layer precursor solution to form a mixture solution; ball milling and drying the mixture solution, and then calcining it in a protective atmosphere.

[0005] Chinese patent application CN103280579A discloses a method for preparing lithium manganese iron phosphate (doped with trace magnesium) as a positive electrode material. The method mainly includes refining and activating the iron oxide raw material, mixing the lithium source, magnesium source, activated iron oxide raw material, manganese source, phosphorus source, and carbon source according to a certain molar stoichiometric coefficient, grinding and refining the precursor in an organic solvent system, pre-calcining, coarse crushing, and sintering.

[0006] In Chinese patents CN105514422B and CN104752715B, oxalate is used as a precipitant to obtain a manganese iron oxalate precursor through an iron-manganese co-precipitation reaction. The iron and manganese elements are first uniformly mixed, and then mixed with a lithium source, a carbon source, and a phosphorus source to synthesize lithium iron manganese phosphate through a high-temperature solid-phase method.

[0007] Summary of the Invention

[0008] The inventors of the present application have found that: in the method disclosed in Chinese patent CN107732176B, the homogenization of transition metal elements is mainly completed by a liquid phase method, which has disadvantages such as difficult control of process parameters, high cost, and difficulty in large-scale production; the high-temperature solid-phase method disclosed in Chinese patent application CN103280579A is simple in process and easy to scale up, but the iron and manganese elements are mixed by high-energy grinding, which makes it difficult to achieve atomic-level mixing. The synthesized lithium iron manganese phosphate material will have uneven iron and manganese micro-regions, which has an adverse effect on the electrochemical performance; although the methods disclosed in Chinese patents CN105514422B and CN104752715B combine the advantages of liquid phase method and high-temperature solid phase method, the precipitation rate of iron and manganese elements in the aqueous solution system is very fast. The particle size of the synthesized manganese iron oxalate is generally large, the distribution of manganese iron elements is uneven, and a mixture of ferrous oxalate and manganese oxalate is easily formed, which has an adverse effect on the electrochemical properties of the lithium iron manganese phosphate positive electrode material, especially the high-temperature cycle performance. In addition, the synthesis process described in the above patents is a one-stage roasting process, in which carbon coating and synthesis reaction are carried out simultaneously. Therefore, free carbon and suspended carbon are likely to appear in the finished product, affecting the carbon coating effect.

[0009] The purpose of the present invention is to overcome the defects of the existing methods for preparing manganese iron oxalate precursors, such as generally large particle size, uneven distribution of manganese iron elements, uneven carbon coating and poor effect, thereby improving the electrochemical performance of lithium manganese iron phosphate positive electrode materials.

[0010] On the one hand, the present invention provides a new method for preparing a manganese ferrooxalate precursor, which comprises the steps of mixing a water-soluble manganese salt, a water-soluble iron salt, an optional water-soluble doping metal salt and a water-soluble oxalate in a water-organic solvent system and performing a coprecipitation reaction.

[0011] On the other hand, the present invention provides a new method for preparing a lithium iron manganese phosphate positive electrode material, which comprises the following steps: mixing a manganese iron oxalate precursor with a lithium source, a phosphorus source and an optional doping aid, drying and then low-temperature calcining to obtain a lithium iron manganese phosphate precursor; then mixing the precursor with an organic carbon source, drying, and high-temperature sintering to obtain a carbon-coated lithium iron manganese phosphate positive electrode material.

[0012] In another aspect, the present invention provides a manganese iron oxalate precursor and a lithium manganese iron phosphate positive electrode material prepared by the above-mentioned novel method.

[0013] The present invention involves co-precipitating manganese, iron, an optional doping metal, and oxalate in a water-organic solvent system. The addition of an organic solvent regulates the co-precipitation process, thereby producing a manganese iron oxalate precursor with controllable dispersion rate, morphology, and particle size, and uniform dispersion of manganese and iron elements. Furthermore, the present invention utilizes a primary low-temperature sintering step to produce a lithium iron manganese phosphate precursor, followed by a secondary high-temperature sintering step to obtain a lithium iron manganese phosphate cathode material free of free carbon and uniformly coated with carbon. This material exhibits high charge-discharge specific capacity and excellent cycling performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG1 is an electron microscope photograph of the organic-water solvent system co-precipitated manganese ferrooxalate precursor obtained in Example 1.

[0015] FIG2 is an electron microscope photograph of the organic-water solvent system co-precipitated manganese ferrooxalate precursor obtained in Example 2.

[0016] FIG3 is an electron microscope photograph of the organic-water solvent system co-precipitated manganese ferrooxalate precursor obtained in Example 3.

[0017] FIG4 is an electron microscope photograph of the corresponding two-stage calcined lithium manganese iron phosphate positive electrode material obtained in Example 1 by co-precipitating the manganese iron oxalate precursor in the organic-water solvent system.

[0018] FIG5 is an electron microscope photograph of the corresponding two-stage calcined lithium manganese iron phosphate positive electrode material obtained in Example 2 by co-precipitating the manganese iron oxalate precursor in the organic-water solvent system.

[0019] FIG6 is an electron microscope photograph of the corresponding two-stage calcined lithium manganese iron phosphate positive electrode material obtained in Example 3 by co-precipitating the manganese iron oxalate precursor in the organic-water solvent system.

[0020] FIG7 is an electron microscope photograph of the manganese ferrooxalate precursor co-precipitated in a single water solvent system obtained in Comparative Example 1.

[0021] FIG8 is an electron microscope photograph of the corresponding two-stage calcined lithium manganese iron phosphate positive electrode material obtained in Example 1 by co-precipitating the manganese iron oxalate precursor with a single water solvent system.

[0022] FIG9 is an electron microscope photograph of the organic-water solvent system co-precipitated manganese ferrooxalate precursor obtained in Comparative Example 2.

[0023] FIG10 is an electron microscope photograph of the lithium manganese iron phosphate positive electrode material obtained in Example 2 by the one-stage sintering process corresponding to the organic-water solvent system co-precipitated manganese iron oxalate precursor.

[0024] FIG11 is an electron microscope photograph of the organic-water solvent system co-precipitated manganese ferrooxalate precursor obtained in Comparative Example 3.

[0025] Figure 12 is the electron microscope photograph of the lithium iron manganese phosphate cathode material obtained by the two-stage calcination process corresponding to the manganese iron oxalate precursor obtained in Comparative Example 3 in an organic-aqueous solvent system.

[0026] Figure 13 is the electron microscope photograph of the manganese iron oxalate precursor obtained in Comparative Example 4 in an organic-aqueous solvent system.

[0027] Figure 14 is the electron microscope photograph of the lithium iron manganese phosphate cathode material obtained by the two-stage calcination process corresponding to the manganese iron oxalate precursor obtained in Comparative Example 4 in an organic-aqueous solvent system.

[0028] Figure 15 is the graph of the average voltage change of the lithium iron manganese phosphate material cycled at 45 °C for 30 weeks in Example 1, Example 2, Example 3, Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4. Detailed implementation mode

[0029] The general formula of the manganese iron oxalate precursor of the present invention is Mn Fe (1-x-z) M z C2O4·yH2O, where both Mn and Fe are divalent, 0 < x < 1, 0 ≤ z < 0.2, 0 ≤ y < 5, and M is an optionally used doped metal. Specific examples of the manganese iron oxalate precursor include, but are not limited to, Mn x Fe (1-x) C2O4·yH2O, where both Mn and Fe are divalent, 0 < x < 1, 0 ≤ y < 5. More specific examples include, but are not limited to, Mn 0.6 Fe 0.4 C2O4·2H2O.

[0030] The new method for preparing the manganese iron oxalate precursor of the present invention includes the steps of mixing a water-soluble manganese salt, a water-soluble iron salt, an optionally used water-soluble M salt, and a water-soluble oxalate in a water-organic solvent system and carrying out a coprecipitation reaction. The organic solvent is preferably an alcohol solvent, more preferably a lower alkanol. Specific examples include, but are not limited to, one or two selected from methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, and n-hexanol. Particularly preferably, it is one or two of methanol and ethanol.

[0031] In the water-organic solvent system, the volume ratio of the organic solvent is preferably 10% - 70%, more preferably 20% - 60%.

[0032] In the present invention, a water-soluble manganese salt is used as the manganese source and a water-soluble iron salt is used as the iron source, preferably divalent manganese salts and iron salts. <00001Specific examples of water-soluble manganese salts include, but are not limited to, one or more of manganous chloride, manganous bromide, manganous nitrate, manganous perchlorate, manganous formate, manganous sulfate, and manganous acetate. Specific examples of water-soluble iron salts include, but are not limited to, one or more of ferrous chloride, ferrous bromide, ferrous fluorosilicate, ferrous nitrate, ferrous perchlorate, ferrous sulfate, ferrous formate, and ferrous acetate.

[0034] Specific examples of the water-soluble oxalate used in the present invention include, but are not limited to, one or more of ammonium oxalate, sodium oxalate, potassium oxalate, and lithium oxalate.

[0035] Specific examples of the water-soluble M salts optionally used in the present invention include, but are not limited to, one or more of magnesium sulfate, nickel sulfate, cobalt sulfate, titanium sulfate, and vanadium sulfate.

[0036] In the coprecipitation reaction, the temperature is usually 0-100° C., preferably 15-50° C.; the reaction time is usually 15-300 min, preferably 30-120 min; and the pH value of the reaction system is 2-14, preferably 5-8.

[0037] The present invention includes a ferromanganese oxalate precursor prepared by the above method.

[0038] The method for preparing a lithium manganese iron phosphate precursor of the present invention comprises the following steps: separating and drying the manganese iron oxalate precursor prepared as described above, mixing it with a lithium source, a phosphorus source and an optional doping aid, drying it and then calcining it at a low temperature to obtain a lithium manganese iron phosphate precursor.

[0039] The present invention includes a lithium manganese iron phosphate precursor prepared by the above method.

[0040] The method for preparing a lithium iron manganese phosphate positive electrode material of the present invention comprises the following steps: mixing a manganese iron oxalate precursor with a lithium source, a phosphorus source and an optional doping aid, drying and then low-temperature calcining to obtain a lithium iron manganese phosphate precursor; mixing the above-mentioned lithium iron manganese phosphate precursor with an organic carbon source, drying, and high-temperature sintering to obtain a carbon-coated lithium iron manganese phosphate positive electrode material.

[0041] In the above-mentioned method for preparing the lithium manganese iron phosphate positive electrode material, it is preferred to use the manganese iron oxalate precursor prepared by the above-mentioned coprecipitation method as the raw material.

[0042] Specific examples of lithium sources include, but are not limited to, one or more of lithium hydroxide, lithium dihydrogen phosphate, lithium acetate, lithium benzoate, lithium bromate, lithium bromide, lithium chlorate, lithium chloride, lithium fluoride, lithium fluorosilicate, lithium formate, lithium iodide, lithium nitrate, lithium perchlorate, lithium tartrate, and lithium carbonate.

[0043] Specific examples of the phosphorus source include, but are not limited to, one or more of lithium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and potassium phosphate.

[0044] Specific examples of the doping aid include, but are not limited to, one or more of titanium dioxide, magnesium carbonate, magnesium hydroxide, niobium pentoxide, nickel hydroxide, and cobalt hydroxide.

[0045] Specific examples of the organic carbon source include, but are not limited to, one or more of glucose, sucrose, lactose, maltose, phenolic resin, carbon nanotubes, and epoxy resin.

[0046] In the above-mentioned methods for preparing the lithium manganese iron phosphate precursor and lithium manganese iron phosphate cathode material, the low-temperature calcination conditions are a calcination temperature of 400-650°C and a calcination time of 1-25 hours. Low-temperature calcination temperatures outside this range can affect the crystallinity, particle morphology, and lithiation effect of the lithium manganese iron phosphate precursor, thereby affecting the performance of the finished product, such as capacity.

[0047] In the above-mentioned method for preparing lithium manganese iron phosphate cathode materials, the high-temperature sintering conditions are a sintering temperature of 700-800°C and a sintering time of 1-25 hours. If the high-temperature sintering temperature does not reach the set range, the carbon coating effect will be affected, thereby affecting product performance.

[0048] The present invention includes a lithium manganese iron phosphate positive electrode material prepared by the above method.

[0049] Example

[0050] The preferred embodiments of the present invention are described below through specific examples, but the present invention is not limited to these examples. Any modifications and changes that do not depart from the spirit of the present invention fall within the scope of the present invention.

[0051] Example 1

[0052] (1) A water-ethanol composite solvent was prepared with ethanol accounting for 30% by volume. 955.26 g of manganous sulfate monohydrate and 1047.24 g of ferrous sulfate heptahydrate were dissolved in 13 L of the water-ethanol composite solvent. 1402.92 g of ammonium oxalate was dissolved in 17 L of the water-ethanol composite solvent. The two solutions were then added dropwise to a base solution containing 5 L of the water-ethanol composite solvent in parallel to perform a coprecipitation reaction. The reaction temperature was 25°C, the reaction time was 60 min, and the reaction pH was 6.5.

[0053] (2) After the coprecipitation reaction is completed, filter and dry to obtain light yellow Mn 0.6 Fe 0.4 1620g of C2O4·2H2O precursor.

[0054] (3) Mn 0.6 Fe 0.41620 g of C2O4·2H2O precursor, 1596.30 g of lithium dihydrogen phosphate, and 6.75 g of titanium dioxide were mixed in a ball mill using pure water as a mixing medium.

[0055] (4) After ball milling, Mn was obtained by spray drying. 0.6 Fe 0.4 A mixture of a C2O4·2H2O precursor, a lithium source, and titanium dioxide is sintered at 450° C. for 10 h to obtain a lithium manganese iron phosphate precursor.

[0056] (5) 500 g of the lithium manganese iron phosphate precursor obtained above and 37.5 g of glucose were mixed in a ball mill using pure water as a mixing medium.

[0057] (6) After ball milling, a mixture of the lithium manganese iron phosphate precursor and the carbon source is obtained by spray drying, and sintered at 750°C for 10 hours to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0058] The electron microscope photographs of the manganese iron oxalate precursor and the lithium manganese iron phosphate positive electrode material obtained in Example 1 are shown in Figures 1 and 4 respectively.

[0059] Example 2

[0060] (1) A water-ethanol composite solvent was prepared with ethanol accounting for 40% by volume. 955.26 g of manganous sulfate monohydrate and 1047.24 g of ferrous sulfate heptahydrate were dissolved in 13 L of the water-ethanol composite solvent. 1402.92 g of ammonium oxalate was dissolved in 17 L of the water-ethanol composite solvent. The two solutions were then added dropwise to a base solution containing 5 L of the water-ethanol composite solvent to perform a coprecipitation reaction. The reaction temperature was 25 °C, the reaction time was 60 min, and the reaction pH was 6.5.

[0061] (2) After the coprecipitation reaction is completed, filter and dry to obtain light yellow Mn 0.6 Fe 0.4 1600g of C2O4·2H2O precursor.

[0062] (3) Mn 0.6 Fe 0.4 1600 g of C2O4·2H2O precursor, 1545.68 g of lithium dihydrogen phosphate, and 6.67 g of titanium dioxide were mixed in a ball mill using pure water as a mixing medium.

[0063] (4) After ball milling, Mn was obtained by spray drying. 0.6 Fe 0.4 A mixture of a C2O4·2H2O precursor, a lithium source, and titanium dioxide is sintered at 500° C. for 10 h to obtain a lithium manganese iron phosphate precursor.

[0064] (5) 500 g of the lithium manganese iron phosphate precursor obtained above and 37.5 g of glucose were mixed in a ball mill using pure water as a mixing medium.

[0065] (6) After ball milling, a mixture of the lithium manganese iron phosphate precursor and the carbon source is obtained by spray drying, and sintered at 800°C for 10 hours to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0066] The electron microscope photographs of the manganese iron oxalate precursor and the lithium manganese iron phosphate positive electrode material obtained in Example 2 are shown in Figures 2 and 5 respectively.

[0067] Example 3

[0068] (1) A water-methanol composite solvent was prepared with a methanol volume ratio of 30%. 955.26 g of manganous sulfate monohydrate and 1047.24 g of ferrous sulfate heptahydrate were dissolved in 13 L of the water-methanol composite solvent. 1402.92 g of ammonium oxalate was dissolved in 17 L of the water-methanol composite solvent. The two solutions were then added dropwise to a base solution containing 5 L of the water-methanol composite solvent in parallel to perform a coprecipitation reaction. The reaction temperature was 25 ° C., the reaction time was 60 min, and the reaction pH was 6.5.

[0069] (2) After the coprecipitation reaction is completed, filter and dry to obtain light yellow Mn 0.6 Fe 0.4 1610g of C2O4·2H2O precursor.

[0070] (3) Mn 0.6 Fe 0.4 1610 g of C2O4·2H2O precursor, 1555.34 g of lithium dihydrogen phosphate, and 6.70 g of titanium dioxide were mixed in a ball mill using pure water as a mixing medium.

[0071] (4) After ball milling, Mn was obtained by spray drying. 0.6 Fe 0.4 A mixture of a C2O4·2H2O precursor, a lithium source, and titanium dioxide is sintered at 550° C. for 10 h to obtain a lithium iron manganese phosphate precursor.

[0072] (5) 500 g of the lithium manganese iron phosphate precursor obtained above and 37.5 g of glucose were mixed in a ball mill using pure water as a mixing medium.

[0073] (6) After ball milling, the mixture of the lithium manganese iron phosphate precursor and the carbon source is obtained by spray drying, and sintered at 800° C. for 10 h to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0074] The electron microscope photographs of the manganese iron oxalate precursor and the lithium manganese iron phosphate positive electrode material obtained in Example 3 are shown in Figures 3 and 6 respectively.

[0075] Comparative Example 1

[0076] (1) 955.26 g of manganous sulfate monohydrate and 1047.24 g of ferrous sulfate heptahydrate were dissolved in 13 L of pure water, and 1402.92 g of ammonium oxalate was dissolved in 17 L of pure water. The two solutions were then added dropwise to a base solution containing 5 L of pure water in parallel to carry out a coprecipitation reaction. The reaction temperature was 25° C., the reaction time was 60 min, and the reaction pH was 6.5.

[0077] (2) After the coprecipitation reaction is completed, filter and dry to obtain light yellow Mn 0.6 Fe 0.4 1605g of C2O4·2H2O precursor.

[0078] (3) Mn 0.6 Fe 0.4 1605 g of C2O4·2H2O precursor, 1550.51 g of lithium dihydrogen phosphate, and 6.69 g of titanium dioxide were mixed in a ball mill using pure water as a mixing medium.

[0079] (4) After ball milling, Mn was obtained by spray drying. 0.6 Fe 0.4 A mixture of a C2O4·2H2O precursor, a lithium source, and titanium dioxide is sintered at 450° C. for 10 h to obtain a lithium manganese iron phosphate precursor.

[0080] (5) 500 g of the lithium manganese iron phosphate precursor obtained above and 37.5 g of glucose were mixed in a ball mill using pure water as a mixing medium.

[0081] (6) After ball milling, a mixture of the lithium manganese iron phosphate precursor and the carbon source is obtained by spray drying, and sintered at 750°C for 10 hours to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0082] The electron microscope photographs of the manganese iron oxalate precursor and the corresponding lithium manganese iron phosphate positive electrode material obtained in Comparative Example 1 are shown in Figures 7 and 8, respectively.

[0083] Comparative Example 2

[0084] (1) A water-ethanol composite solvent was prepared with ethanol accounting for 30% by volume. 955.26 g of manganous sulfate monohydrate and 1047.24 g of ferrous sulfate heptahydrate were dissolved in 13 L of the water-ethanol composite solvent. 1402.92 g of ammonium oxalate was dissolved in 17 L of the water-ethanol composite solvent. The two solutions were then added dropwise to a base solution containing 5 L of the water-ethanol composite solvent to perform a coprecipitation reaction. The reaction temperature was 25 °C, the reaction time was 60 min, and the reaction pH was 6.5.

[0085] (2) After the coprecipitation reaction is completed, filter and dry to obtain light yellow Mn 0.6 Fe0.4 1590g of C2O4·2H2O precursor.

[0086] (3) Mn 0.6 Fe 0.4 1590 g of C2O4·2H2O precursor was mixed with 1580 g of lithium dihydrogen phosphate, 153 g of glucose, and 6.63 g of titanium dioxide in a ball mill, using pure water as a mixing medium.

[0087] (4) After ball milling, Mn was obtained by spray drying. 0.6 Fe 0.4 A mixture of C2O4·2H2O precursor, lithium source, carbon source and titanium dioxide was sintered at 750°C for 10 hours to obtain lithium manganese iron phosphate positive electrode material.

[0088] The electron microscope photographs of the manganese iron oxalate precursor and the lithium manganese iron phosphate positive electrode material obtained in Comparative Example 2 are shown in Figures 9 and 10, respectively.

[0089] Comparative Example 3

[0090] (1) A water-ethanol composite solvent was prepared with an ethanol volume ratio of 5%. 955.26 g of manganous sulfate monohydrate and 1047.24 g of ferrous sulfate heptahydrate were dissolved in 13 L of the water-ethanol composite solvent. 1402.92 g of ammonium oxalate was dissolved in 17 L of the water-ethanol composite solvent. The two solutions were then added dropwise to a base solution containing 5 L of the water-ethanol composite solvent in parallel to perform a coprecipitation reaction. The reaction temperature was 25°C, the reaction time was 60 min, and the reaction pH was 6.5.

[0091] (2) After the coprecipitation reaction is completed, filter and dry to obtain light yellow Mn 0.6 Fe 0.4 1609g of C2O4·2H2O precursor.

[0092] (3) Mn 0.6 Fe 0.4 1609 g of C2O4·2H2O precursor, 1585.46 g of lithium dihydrogen phosphate, and 6.70 g of titanium dioxide were mixed in a ball mill using pure water as a mixing medium.

[0093] (4) After ball milling, Mn was obtained by spray drying. 0.6 Fe 0.4 A mixture of a C2O4·2H2O precursor, a lithium source, and titanium dioxide is sintered at 450° C. for 10 h to obtain a lithium manganese iron phosphate precursor.

[0094] (5) 500 g of the lithium manganese iron phosphate precursor obtained above and 37.5 g of glucose were mixed in a ball mill using pure water as a mixing medium.

[0095] (6) After ball milling, a mixture of the lithium manganese iron phosphate precursor and the carbon source is obtained by spray drying, and sintered at 750°C for 10 hours to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0096] The electron microscope photographs of the manganese iron oxalate precursor and the lithium manganese iron phosphate positive electrode material obtained in Comparative Example 3 are shown in Figures 11 and 12, respectively.

[0097] Comparative Example 4

[0098] (1) A water-ethanol composite solvent was prepared with ethanol accounting for 80% by volume. 318.42 g of manganous sulfate monohydrate and 349.08 g of ferrous sulfate heptahydrate were dissolved in 13 L of the water-ethanol composite solvent. 467.64 g of ammonium oxalate was dissolved in 17 L of the water-ethanol composite solvent. The two solutions were then added dropwise to a base solution containing 5 L of the water-ethanol composite solvent in parallel to perform a coprecipitation reaction. The reaction temperature was 25° C., the reaction time was 60 min, and the reaction pH was 6.5.

[0099] (2) After the coprecipitation reaction is completed, filter and dry to obtain light yellow Mn 0.6 Fe 0.4 1625g of C2O4·2H2O precursor.

[0100] (3) Mn 0.6 Fe 0.4 1625 g of C2O4·2H2O precursor, 1601.23 g of lithium dihydrogen phosphate, and 6.77 g of titanium dioxide were mixed in a ball mill using pure water as a mixing medium.

[0101] (4) After ball milling, Mn was obtained by spray drying. 0.6 Fe 0.4 A mixture of a C2O4·2H2O precursor, a lithium source, and titanium dioxide is sintered at 450° C. for 10 h to obtain a lithium manganese iron phosphate precursor.

[0102] (5) 500 g of the lithium manganese iron phosphate precursor obtained above and 37.5 g of glucose were mixed in a ball mill using pure water as a mixing medium.

[0103] (6) After ball milling, a mixture of the lithium manganese iron phosphate precursor and the carbon source is obtained by spray drying, and sintered at 750°C for 10 hours to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0104] The electron microscope photographs of the manganese iron oxalate precursor and the lithium manganese iron phosphate positive electrode material obtained in Comparative Example 4 are shown in Figures 13 and 14, respectively.

[0105] The performance testing methods of the precursors and manganese iron phosphate cathode materials in the examples and comparative examples are as follows:

[0106] Particle size test: laser particle size analyzer;

[0107] Manganese content test: ammonium ferrous sulfate titration method;

[0108] Iron content test: redox titration method;

[0109] Electrochemical performance testing: Button test system, CR2430 button cell. The specific operation is as follows: The obtained positive electrode material, acetylene black, and polyvinylidene fluoride (PVDF) were added to N-methylpyrrolidone (NMP) in a mass ratio of 90:5:5. The mixture was stirred and mixed for 30 minutes using a planetary mixer to form a uniform and stable slurry. The slurry was evenly coated on aluminum foil using a fully automatic coating machine. The electrode sheet was dried, rolled, and punched into a raw sheet. The button cell was assembled in a high-purity argon glove box and then tested. The electrochemical performance was tested using a blue electric test system. The charge and discharge range was 2.0-4.3V, and the nominal specific capacity was 154mAh / g.

[0110] Table 1 Performance indicators of manganese iron oxalate and lithium manganese iron phosphate in Examples 1-3 and Comparative Examples 1-4

[0111] It can be seen from Figures 1, 2, 3, 7, 9, 11 and 13 that when the proportion of the organic solvent in the water-organic solvent system is within the range described in the present invention, the primary particle size of the precipitated manganese iron oxalate precursor is significantly smaller and the morphology is more uniform. When the proportion of a single aqueous solvent or organic solvent is not within the range described in the present invention, the primary particle size of the manganese iron oxalate precursor is significantly larger and the morphology is non-uniform. It can be seen from the red area in Figure 10 that there are free manganese iron phosphate products in the finished product after calcination. As can be seen from Figures 4, 5, 6, 8, 12 and 14, no free carbon was found in the two-stage calcined sample, and the carbon was coated on the surface of the material in the form of a film; it can be concluded from the data of Examples 1-3 and Comparative Examples 1-4 that when the proportion of the organic solvent in the water-organic solvent system is within the range described in the present invention, the manganese-iron ratio of the obtained manganese iron oxalate precursor is basically consistent with the designed value, and a lithium manganese iron phosphate positive electrode material with excellent carbon coating effect can be obtained through the two-stage calcination, and the charge and discharge specific capacity is better than that of the comparative example.

[0112] As shown in Figure 15, after 50 weeks of high-temperature cycle testing at 45°C, the voltage drop of the lithium manganese iron phosphate positive electrode materials in Examples 1, 2, and 3 is significantly lower than that in Comparative Example 1, 2, 3, and 4, and Comparative Example 2 is lower than that in Comparative Example 1, 3, and 4.

[0113] The present invention regulates the synthesis process of the manganese iron oxalate precursor by introducing a composite solvent in a reasonable proportion, thereby obtaining a precursor with small particle size, regular morphology, and appropriate iron-manganese ratio. Then, through a two-stage sintering process, a manganese iron phosphate lithium positive electrode material with uniform carbon coating, high discharge specific capacity, good cycle performance, and stable voltage is synthesized.

Claims

1. A method for preparing a manganese-iron oxalate precursor, the general formula of the manganese-iron oxalate precursor being Mn x Fe (1-x-z) M z C2O4·yH2O, where both Mn and Fe are divalent, 0 < x < 1, 0 ≤ z < 0.2, 0 ≤ y < 5, and M is an optionally used doping metal cation selected from one or more of magnesium, nickel, cobalt, titanium, and vanadium. The method comprises the following steps: A water-soluble manganese salt, a water-soluble iron salt, an optional water-soluble M salt and a water-soluble oxalate are mixed in a water-organic solvent system to carry out a coprecipitation reaction.

2. The method of claim 1, wherein: The organic solvent is an alcohol organic solvent, more preferably a lower alkanol, preferably one or more of methanol, ethanol, isopropanol, ethylene glycol, n-butanol, isobutanol, and n-hexanol, more preferably one or two of methanol and ethanol.

3. The method according to claim 1 or 2, wherein: In the water-organic solvent system, the volume ratio of the organic solvent is 10%-70%, preferably 20%-60%.

4. The method according to any one of claims 1 to 3, wherein: The water-soluble manganese salt is one or more of manganous chloride, manganous bromide, manganous nitrate, manganous perchlorate, manganous formate, manganous sulfate and manganous acetate; the water-soluble iron salt is one or more of ferrous chloride, ferrous bromide, ferrous fluorosilicate, ferrous nitrate, ferrous perchlorate, ferrous sulfate, ferrous formate and ferrous acetate; the water-soluble M salt is one or more of magnesium sulfate, nickel sulfate, cobalt sulfate, titanium sulfate and vanadium sulfate; the water-soluble oxalate is one or more of ammonium oxalate, sodium oxalate, potassium oxalate and lithium oxalate.

5. The method according to any one of claims 1 to 4, wherein: The coprecipitation reaction temperature is 0-100° C., preferably 15-50° C., the preferred reaction time is 15-300 min, preferably 30-120 min, and the preferred pH value of the reaction system is 2-14, preferably 5-8.

6. A method as claimed in any one of claims 1 to 5, wherein the mixing is carried out by dissolving a water-soluble manganese salt and a water-soluble iron salt and an optional water-soluble M salt together in a water-organic solvent system, dissolving a water-soluble oxalate in a water-organic solvent system, and then adding the two solutions to the water-organic solvent system.

7. A ferromanganese oxalate precursor prepared by the method according to any one of claims 1 to 6.

8. A method for preparing a lithium manganese iron phosphate precursor, the method comprising the following steps: Prepare a ferromanganese oxalate precursor by the method according to any one of claims 1 to 6, separate and dry; The manganese iron oxalate precursor is mixed with a lithium source, a phosphorus source and an optional doping aid, dried and then calcined at a low temperature to obtain a manganese iron phosphate lithium precursor.

9. A method for preparing a lithium manganese iron phosphate positive electrode material, the method comprising the following steps: Mix the manganese iron oxalate precursor with a lithium source, a phosphorus source, and an optionally used doping assistant, dry it, and then perform low-temperature calcination to obtain a lithium iron manganese phosphate precursor, wherein the general formula of the manganese iron oxalate precursor is Mn x Fe (1-x-z) M z C2O4·yH2O, where both Mn and Fe are divalent, 0 < x < 1, 0 ≤ z < 0.2, 0 ≤ y < 5, and M is an optionally used doping metal cation selected from one or more of magnesium, nickel, cobalt, titanium, and vanadium; The lithium manganese iron phosphate precursor is mixed with an organic carbon source, dried, and sintered at a high temperature to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

10. The method for preparing lithium manganese iron phosphate positive electrode material as claimed in claim 9, wherein the manganese iron oxalate precursor is prepared by the method as claimed in any one of claims 1 to 6.

11. The method for preparing lithium manganese iron phosphate positive electrode material according to any one of claims 8 to 10, wherein: The lithium source is one or more of lithium hydroxide, lithium dihydrogen phosphate, lithium acetate, lithium benzoate, lithium bromate, lithium bromide, lithium chlorate, lithium chloride, lithium fluoride, lithium fluorosilicate, lithium formate, lithium iodide, lithium nitrate, lithium perchlorate, lithium tartrate, and lithium carbonate; the phosphorus source is one or more of lithium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and potassium phosphate; the doping aid is one or more of titanium compounds, magnesium compounds, niobium compounds, vanadium compounds, nickel compounds, and cobalt compounds; and the organic carbon source is one or more of glucose, sucrose, lactose, maltose, phenolic resin, carbon nanotubes, and epoxy resin.

12. The method for preparing lithium manganese iron phosphate positive electrode material according to any one of claims 8 to 11, wherein: The low-temperature calcination conditions are a calcination temperature of 400-650° C. and a calcination time of 1-25 hours.

13. The method for preparing lithium manganese iron phosphate positive electrode material according to any one of claims 8 to 12, wherein: The high temperature sintering conditions are a sintering temperature of 700-800° C. and a sintering time of 1-25 hours.

14. Lithium manganese iron phosphate positive electrode material prepared by the method according to any one of claims 9 to 13.

Images