Preparation method of high-cycle high-rate lithium manganese iron phosphate positive electrode material

Abstract

The invention provides a preparation method of a high-cycle and high-rate lithium manganese iron phosphate positive electrode material, and relates to the technical field of preparation of lithium ion battery positive electrode materials, and the preparation method comprises the following steps: S1, preparing an iron-manganese solution by using ferrous salt, bivalent manganese salt and first doped metal salt as raw materials, mixing the iron-manganese solution with a precipitator, and reacting to obtain an iron-manganese solid solution intermediate; s2, sintering in an oxidizing atmosphere to obtain a precursor; s3, dissolving the precursor, a lithium source, a phosphorus source, a first carbon source and a second doped metal salt in pure water, grinding, spraying, drying and sintering to obtain first lithium manganese iron phosphate; s4, dissolving the first lithium manganese iron phosphate, a second carbon source and a third doped metal salt in pure water, grinding, spraying, drying and sintering to obtain a lithium manganese iron phosphate positive electrode material; when the precursor prepared by the method is used for preparing the lithium manganese iron phosphate positive electrode material of the lithium battery, since the structure of the precursor has stability, synthesis conditions do not need to be strictly controlled, and the accuracy of ingredients such as a lithium source, a phosphorus source, a carbon source and a doping agent is enhanced.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion battery positive electrode material preparation, and in particular to a method for preparing a high-cycle and high-rate lithium iron manganese phosphate positive electrode material. Background Art

[0002] Lithium manganese iron phosphate (LMFP) and lithium iron phosphate (LFP) are both polyanion cathode materials with an olivine structure. Through the synergistic effect of Mn and Fe, the voltage platform of lithium manganese iron phosphate can jump from 3.4V of lithium iron phosphate to 4.1V, thereby achieving a significant increase in energy density of 15% to 20%. In addition, LMFP also performs better than LFP in low temperature environments. Its capacity retention rate at -20°C can reach 75%, which is significantly higher than the capacity retention rate of LFP (60% to 70%). Therefore, as an upgraded alternative to LFP, LMFP shows broad application potential in the market.

[0003] In terms of preparation process, LMFP and LFP have similarities. Among them, the solid phase method, as the most traditional method, has the advantages of simple process and easy mass production, but it also has the problem of segregation of ferromanganese materials, making it difficult to form a uniform solid solution, resulting in poor cycle performance. In addition, the inherent low ionic and electronic conductivity of lithium iron manganese phosphate seriously affects the performance of its rate performance, which increases the difficulty of industrial production of lithium iron manganese phosphate, making the balance between material performance and cost particularly difficult. Therefore, how to suppress the segregation of ferromanganese materials, ensure the uniform distribution of Mn and Fe at the atomic scale, and improve the conductivity of the material to improve its cycle performance and rate performance has become a technical problem that technicians in this field urgently need to overcome.

[0004] In order to solve the above problems, some studies have tried to prepare ferromanganese solid solution in advance and then perform solid phase reaction to ensure the uniformity of its element distribution. For example, the patent application with publication number CN115676794A proposes a method for preparing lithium iron manganese phosphate positive electrode material by coprecipitation, which uses the characteristics of ferrous ions, manganese ions and carbonate precipitation Ksp values ​​to prepare a small-particle ferrous manganese carbonate precursor, and realizes the uniform mixing of iron and manganese at the atomic level. Phosphorus pentoxide and ferrous manganese carbonate are then eutectic to generate ferrous manganese pyrophosphate, and then the pyrophosphate is hydrolyzed to generate phosphate by low pH hydrothermal reaction, and finally lithium iron manganese phosphate is generated. However, the process of this method is cumbersome, involving steps such as eutectic and hydrothermal reaction, which is not conducive to industrial production. The patent application with publication number CN118561328A proposes a method of using solid carbonate as a precipitant, and the local carbonate in the reaction solution is supersaturated by a one-time instantaneous addition under an inert atmosphere, thereby rapidly nucleating and improving the particle size uniformity of ferromanganese coprecipitation. The patent application with publication number CN1114394584A proposes a method for preparing lithium iron manganese phosphate battery materials by coprecipitation-solid phase combination, which is characterized in that Mn is synthesized by coprecipitation. x Fe 1-x C2O4·2H2O precursor, then ball milled using a planetary ball mill, dried and sintered in two steps to synthesize lithium iron manganese phosphate / carbon (LiMn x Fe 1-x PO4 / C) cathode material. It is worth noting that the divalent iron manganese precursor obtained by the above method is not stable enough in the air. It is necessary to strictly control the oxidation degree of the precursor or make it carry crystal water to enhance its stability during the synthesis process, otherwise it will affect the accuracy and phase purity of the synthetic ingredients of lithium manganese iron phosphate, which undoubtedly increases the difficulty and cost of industrialization. Summary of the invention

[0005] Based on the above background, the present invention aims to provide a preparation method of lithium manganese iron phosphate by coprecipitation combined with calcination to obtain an iron manganese oxide precursor, and then sintering and composite doping to obtain lithium manganese iron phosphate, so as to effectively improve the rate performance and cycle stability of lithium manganese iron phosphate, as follows: A method for preparing a high-cycle and high-rate lithium manganese iron phosphate positive electrode material comprises the following steps: S1, preparing an iron-manganese solution using ferrous salt, divalent manganese salt and a first doping metal salt as raw materials, mixing with a precipitant, reacting and aging, filtering, washing and drying to obtain an iron-manganese solid solution intermediate; S2, sintering the iron-manganese solid solution intermediate obtained in step S1 in an oxidizing atmosphere to obtain a pure-phase iron-manganese oxide precursor having uniform distribution of manganese and iron at the atomic level; the total content of the first doping metal in the iron-manganese oxide precursor is 0-10000 mass ppm (based on the total mass of the iron-manganese oxide precursor); S3, dissolving the iron manganese oxide precursor, the lithium source, the phosphorus source, the first carbon source, and the second doping metal salt in pure water, wet grinding to a target particle size, spraying, drying, and sintering to obtain a first lithium manganese iron phosphate; S4, dissolving the first lithium manganese iron phosphate, the second carbon source and the third doped metal salt in pure water in step S3, grinding to a target particle size by wet method, spraying, drying and sintering to obtain a second lithium manganese iron phosphate, i.e., a lithium manganese iron phosphate positive electrode material.

[0006] Preferably, in step S1: The ferrous salt is one or more of ferrous sulfate, ferrous chloride, ferrous oxalate, ferrous nitrate, ferrous acetate and hydrates thereof; The divalent manganese salt is one or more of manganese sulfate, manganese chloride, manganese oxalate, manganese nitrate, manganese acetate and hydrates thereof; The precipitant is one or more of oxalic acid, ammonium oxalate, ammonium hydrogen oxalate, sodium oxalate, sodium hydrogen oxalate, potassium oxalate, potassium hydrogen oxalate, ammonium carbonate, ammonium hydrogen carbonate, sodium carbonate, sodium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, sodium hydroxide, potassium hydroxide, and ammonia water; The first doping metal salt is one or more of vanadium salt, magnesium salt, nickel salt, cobalt salt, titanium salt, copper salt, niobium salt, chromium salt, zinc salt, molybdenum salt and tungsten salt.

[0007] Preferably, in step S1: The concentration of the ferrous ions is 0.15-1.98 mol / L, and the concentration of the divalent manganese ions is 0.22-1.35 mol / L; In terms of molar amount, the molar ratio of ferrous ion to divalent manganese ion is 1:9 to 9:1; The molar ratio of the amount of the precipitant added to Mn²⁺+Fe²⁺ is 1.0-2.5:1.

[0008] Preferably, in step S1: The reaction time is 0.1-5 hours and the temperature is 20-90° C.; the aging time is 0.5-20 hours; the drying temperature is 50-120° C. and the drying duration is 4-24 hours.

[0009] Preferably, in step S2: the sintering temperature is 500-900° C., and the sintering time is 3-10 hours.

[0010] Preferably, in step S3: The lithium source is one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate, lithium oxalate, and lithium acetate; The phosphorus source is one or more of phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and lithium phosphate; The first carbon source is one or more of glucose, sucrose, fructose, starch, PEG, PVP, cyclodextrin, and citric acid; The second doping metal salt is one or more of vanadium salt, magnesium salt, nickel salt, cobalt salt, titanium salt, copper salt, niobium salt, chromium salt, zinc salt, molybdenum salt and tungsten salt.

[0011] Preferably, in step S3: The molar ratio of the lithium source, the iron source + manganese source of the iron manganese oxide precursor, the second doping metal salt, and the phosphorus source is 1.01-1.06:0.92-1.02:0-0.08:1; The mass ratio of the first carbon source to the iron manganese oxide precursor is 0.05-0.15:1; The target particle size of the wet grinding is D50 = 0.03-0.06 microns; During sintering, the sintering temperature is 350-680°C, the sintering time is 3-10 hours, and the heating rate is 0.5-5.0°C / min; The solid content in the reaction system is 30-50wt%.

[0012] Preferably, in step S4: The third doping metal salt is one or more of vanadium salt, magnesium salt, nickel salt, cobalt salt, titanium salt, copper salt, niobium salt, chromium salt, zinc salt, molybdenum salt and tungsten salt; The mass ratio of the first lithium manganese iron phosphate to the third doping metal salt is 1:0-0.01; The feeding mass ratio of the second carbon source to the first lithium manganese iron phosphate is 0.05-0.15:1; The target particle size of the wet grinding is D50 = 0.2-0.6 microns; The solid content in the reaction system is 15-45wt%.

[0013] Preferably, in step S4: During sintering, the sintering temperature is 650-800° C., the sintering time is 3-10 hours, and the heating rate is 0.5-5.0° C. / min.

[0014] Preferably, in step S4: The second carbon source is one or more of glucose, sucrose, fructose, citric acid, phenolic resin, polyvinyl alcohol, polyethylene glycol, polyvinyl pyrrolidone, starch, carbon black, acetylene black, graphite, graphene, fullerene, and conductive carbon tube; The total amount of doped metal in the second lithium manganese iron phosphate is 5000-20000 mass ppm.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. When the iron-manganese oxide precursor prepared by the present invention is used as a precursor to prepare lithium iron manganese phosphate positive electrode materials for lithium batteries, due to its stable structure, there is no need to strictly control the synthesis conditions, which enhances the accuracy of ingredients such as lithium source, phosphorus source, carbon source, dopant, etc., and is conducive to industrialization.

[0016] 2. The iron manganese oxide prepared by the present invention is a pure phase iron manganese solid solution precursor, which has a small primary particle size, better uniformity, and better sphericity. The synthesized lithium iron manganese phosphate ensures uniform distribution of manganese iron, reduces manganese segregation, reduces manganese dissolution, and is beneficial to improving cycle stability.

[0017] 3. The finished product of manganese iron manganese oxide in the present invention is nano-sized after step-by-step sintering and multi-element composite doping, and the conductivity is significantly improved, which can show better rate performance.

[0018] 4. The iron-manganese solid solution intermediate prepared by the co-precipitation method of the present invention forms a pure-phase iron-manganese oxide after sintering in an oxidizing atmosphere, ensuring that Mn / Fe is evenly distributed at the atomic scale, avoiding the segregation problem of the traditional solid phase method. At the same time, the uniform Mn / Fe distribution reduces the phase change stress during the charge and discharge process, inhibits the dissolution of manganese, and thus greatly improves the cycle life.

[0019] 5. The present invention introduces the first, second and third doping metal salts step by step to construct a multi-doping system based on atomic-level mixing, which not only enhances the stability of the crystal structure, but also further broadens the lithium ion diffusion channel through the synergistic effect between metals, shortens the diffusion distance of lithium ions, and improves the electrical conductivity of the material.

[0020] 6. The present invention adopts a two-step sintering process and uses a step-by-step composite coating strategy of multiple carbon sources. Compared with one-step sintering, it effectively improves the uniformity of carbon layer coating, reduces the generation of floating carbon, and improves the degree of graphitization of the carbon layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a SEM image of the pure phase iron manganese oxide precursor prepared in Example 1 of the present invention; Figure 2 XRD spectrum of the pure phase iron manganese oxide precursor prepared in Example 1 of the present invention; Figure 3 This is a SEM image of the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0022] Example 1 This embodiment uses undoped iron manganese oxide to dope Ti / Mg / V in two steps to prepare Mn60 lithium manganese iron phosphate, including the following steps: (1) Dissolve ferrous sulfate and manganese sulfate in water in turn to prepare a total concentration of ferrous sulfate and divalent manganese of 2 mol / L and Mn2+ :Fe 2+ =6:4 uniform iron-manganese liquid; the iron-manganese liquid is heated to 60°C, according to (Mn 2+ +Fe 2+ ):CO3 2- =1:1.1 ratio, weigh ammonium carbonate solid, prepare a 2mol / L solution, add it to the iron-manganese solution, react for 2h, stand for 1h, filter, wash and dry at 90℃ for 16h to obtain an iron-manganese solid solution intermediate; sinter the iron-manganese solid solution intermediate at 700℃ in an oxidizing atmosphere for 6h to obtain a pure phase iron-manganese oxide precursor. Its SEM and XRD spectra are shown in Figures 1 and 2 respectively. It can be seen from the figure that the obtained powder is a pure phase nano-scale (FeMn)2O3 without other iron-manganese oxide impurities, and the average primary particle size is about 50nm. This nano-scale iron-manganese precursor of iron-manganese solid solution has significant advantages such as high processing efficiency and good cycle performance.

[0023] (2) The iron manganese oxide precursor is mixed with ammonium dihydrogen phosphate, lithium carbonate, magnesium oxide, vanadium pentoxide, glucose and deionized water according to the element molar ratio of Li: (Mn + Fe): Mg: V: P = 1.02: 0.95: 0.02: 0.02: 1, the mass ratio of carbon source to iron manganese oxide precursor is 0.08: 1, the solid content is controlled to 40%, the mixture is sand-milled to D50 = 0.05 microns, sprayed and dried, and the resulting product is heated to 500 ° C at a rate of 2 ° C / min under an inert atmosphere, and kept warm for 2 hours to obtain the first manganese iron lithium phosphate. The first manganese iron lithium phosphate is dissolved in pure water with titanium oxide, glucose, and polyethylene glycol according to the mass ratio of LMFP: Ti: GLC: PEG = 1: 0.006: 0.05: 0.05, the solid content is controlled to 30%, and the mixture is sand-milled to D 50 = 0.3 micron, sprayed and dried, the obtained product was heated to 750°C at a rate of 2.5°C / min in an inert atmosphere, and kept warm for 8 hours to obtain a lithium manganese iron phosphate positive electrode material, whose SEM image is as follows Figure 3 As shown in the figure, it can be seen that the prepared carbon-coated lithium manganese iron phosphate powder (lithium manganese iron phosphate positive electrode material) has round primary particles and no obvious floating carbon; the average particle size is only 100nm-150nm, which is beneficial to shorten the diffusion distance of lithium ions and improve the deintercalation kinetics of lithium ions.

[0024] The button cell was prepared using the lithium manganese iron phosphate positive electrode material prepared in this embodiment. The charging and discharging behavior of the button cell was tested and analyzed by the blue electric test system using the constant current-constant voltage charging (CC-CV) and constant current discharge (DC) test methods. The 1C discharge capacity was 143.5mAh / g, the 1C constant current charge ratio was 85.2%, and the capacity retention rate after 50 cycles of 1C was 99.8%. The powder compaction test equipment of Sansi was used, and the compaction density was 2.28g / cc at a pressure of 3T.

[0025] Example 2 This embodiment uses undoped iron manganese oxide to dope Nb / Mg / V in two steps to prepare Mn55 lithium manganese iron phosphate, including the following steps: (1) Dissolve ferrous oxalate and manganese oxalate in water in turn to prepare a total concentration of ferrous iron and divalent manganese of 1.5 mol / L and Mn 2+ :Fe 2+ =5:5 uniform iron-manganese liquid; heat the iron-manganese liquid to 40°C, press (Mn 2+ +Fe 2+ ):NH3·H2O=1:2.2 ratio of ammonia water, reaction time 3h, standing aging for 6h, filtering, washing and drying at 80°C for 16h to obtain an iron-manganese solid solution intermediate; sintering the iron-manganese solid solution intermediate at 850°C for 4h to obtain a pure phase iron-manganese oxide precursor.

[0026] (2) Mix ferrous manganese oxide with lithium dihydrogen phosphate, lithium carbonate, magnesium sulfate, ammonium metavanadate, sucrose and deionized water in an element molar ratio of Li:(Mn+Fe):Mg:V:P = 1.015:0.94:0.02:0.02:1, a mass ratio of carbon source to ferrous manganese oxide precursor of 0.06:1, and control the solid content to 45%, and grind the mixture to D 50 =0.045 microns, sprayed and dried, the resulting product was heated to 525°C at a rate of 2°C / min in an inert atmosphere, and kept warm for 2 hours to obtain the first lithium manganese iron phosphate. The first lithium manganese iron phosphate was mixed with niobium oxalate, sucrose, and polyethylene glycol in a mass ratio of LMFP: Nb: SUC: PEG = 1: 0.008: 0.04: 0.08, the solid content was controlled to 30wt%, and the mixture was sand-milled to D 50 =0.32 micron, spray and dry, and the obtained product is heated to 780°C at a rate of 3°C / min under an inert atmosphere and kept warm for 4 hours to obtain lithium manganese iron phosphate positive electrode material.

[0027] The button cell was prepared using the lithium manganese iron phosphate positive electrode material prepared in this embodiment. The charging and discharging behavior of the button cell was tested and analyzed by the blue electric test system using the constant current-constant voltage charging (CC-CV) and constant current discharge (DC) test methods. The 1C discharge capacity was 147.7mAh / g, the 1C constant current charge ratio was 90.1%, and the capacity retention rate after 50 cycles of 1C was 99.9%. The powder compaction test equipment of Sansi was used, and the compaction density was 2.32g / cc at a pressure of 3T.

[0028] Example 3 In this embodiment, Mg-doped manganese iron oxide is doped with Ti / V in one step to prepare Mn60 lithium manganese iron phosphate, including the following steps: Dissolve ferrous acetate, manganese acetate, and magnesium acetate in water in turn to prepare a total concentration of ferrous acetate, divalent manganese, and divalent magnesium of 2 mol / L, Mn 2+ :Fe 2+ Mg 2+ =6:4:0.2 iron-manganese liquid; heat the iron-manganese liquid to 60°C, according to (Mn 2+ +Fe 2+ ): CO3 2- :OH - =1:1:0.1 ratio of ammonium carbonate and sodium hydroxide, after preparing a precipitant of 2 mol / L, add it to the iron manganese solution, react for 2 hours, let it stand for 1 hour, filter and wash it, and then dry it at 120°C for 10 hours to obtain an iron manganese solid solution intermediate; sinter the iron manganese solid solution intermediate at 750°C for 6 hours to obtain a Mg-doped iron manganese oxide precursor.

[0029] Mg-doped iron manganese oxide was mixed with phosphoric acid, lithium carbonate, titanium oxide, ammonium metavanadate, glucose and deionized water according to the element molar ratio of Li: (Mn+Fe+Mg): Ti: V: P = 1.02: 0.97: 0.02: 0.02: 1, the mass ratio of glucose to iron manganese oxide precursor was 0.06:1, the solid content was controlled to 40wt%, and the mixture was sand-milled to D 50 =0.055 microns, sprayed and dried, the obtained product was heated to 580°C at a rate of 5°C / min in an inert atmosphere, and kept warm for 6 hours to obtain the first lithium manganese iron phosphate. The first lithium manganese iron phosphate was mixed with glucose and polyethylene glycol at a mass ratio of 1:0.06:0.03, the solid content was controlled to 30wt%, and the mixture was sand-milled to D50=0.35 microns, sprayed and dried, and the obtained product was heated to 750°C at a rate of 2.5°C / min in an inert atmosphere, and kept warm for 10 hours to obtain the lithium manganese iron phosphate positive electrode material.

[0030] The button cell was prepared using the lithium manganese iron phosphate positive electrode material prepared in this embodiment. The charging and discharging behavior of the button cell was tested and analyzed by the blue electric test system using the constant current-constant voltage charging (CC-CV) and constant current discharge (DC) test methods. The 1C discharge capacity was 145.8mAh / g, the 1C constant current charge ratio was 89.2%, and the capacity retention rate after 50 cycles of 1C was 99.6%. The powder compaction test equipment of Sansi was used, and the compaction density was 2.28g / cc at a pressure of 3T.

[0031] Example 4 This embodiment uses Mg-doped manganese iron oxide to dope Mo / Ti in two steps to prepare Mn60 lithium manganese iron phosphate, including the following steps: (1) Dissolve ferrous nitrate, manganese nitrate and magnesium sulfate in water in turn to prepare a total concentration of ferrous nitrate, divalent manganese and divalent magnesium of 2 mol / L. 2+ :Fe 2+ Mg 2+ =6:4:0.2 iron-manganese liquid; heat the iron-manganese liquid to 50°C, according to (Mn 2+ +Fe 2+ ):OH - :CO3 2- =1:2:0.1 ratio of sodium hydroxide and sodium carbonate, after preparing a precipitant of 2 mol / L, add it to the iron manganese solution, react for 1 hour, let it stand for aging for 3 hours, filter and wash it, and then dry it at 90°C for 10 hours to obtain an iron manganese solid solution intermediate; sinter the iron manganese solid solution intermediate at 780°C for 6 hours to obtain a doped iron manganese oxide precursor.

[0032] (2) Mix Mg-doped iron manganese oxide with ammonium dihydrogen phosphate, lithium carbonate, ammonium molybdate tetrahydrate, sucrose and deionized water in an element molar ratio of Li: (Mn+Fe+Mg): Mo: P = 1.02: 0.97: 0.03: 1, and a mass ratio of sucrose to iron manganese oxide precursor of 0.09:1. The solid content is controlled to 40wt%, and the mixture is sand-milled to D 50 =0.045 micron, spray and dry, the obtained product is heated to 600°C at a rate of 5°C / min in an inert atmosphere, and kept warm for 6 hours to obtain the first lithium manganese iron phosphate. The first lithium manganese iron phosphate is mixed with titanium tetrachloride, glucose, and polyvinyl alcohol in a mass ratio of LMFP: Ti: GLC: PVP = 1:0.03:0.06:0.03, the solid content is controlled to 32wt%, and the mixture is sand-milled to D 50 =0.35 micron, spray and dry, and the obtained product is heated to 750°C at a rate of 2°C / min under an inert atmosphere and kept warm for 8h to obtain lithium manganese iron phosphate positive electrode material.

[0033] The button cell was prepared using the lithium manganese iron phosphate positive electrode material prepared in this embodiment. The charging and discharging behavior of the button cell was tested and analyzed by the blue electric test system using the constant current-constant voltage charging (CC-CV) and constant current discharge (DC) test methods. The 1C discharge capacity was 147.5mAh / g, the 1C constant current charge ratio was 90.2%, and the capacity retention rate after 50 cycles of 1C was 99.9%. The powder compaction test equipment of Sansi was used, and the compaction density was 2.28g / cc at a pressure of 3T.

[0034] Comparative Example 1 The difference between this comparative example and Example 1 is that a mixed phase of iron-manganese oxides, namely Mn3O4 and Fe2O3, is used as a precursor, and the remaining steps are the same as those in Example 1.

[0035] The button cell was prepared using the lithium iron manganese phosphate positive electrode material prepared in this comparative example. The charge and discharge behavior of the button cell was tested and analyzed using the blue electric test system using constant current-constant voltage charging (CC-CV) and constant current discharge (DC) test methods. The 1C discharge capacity was 139.9mAh / g, the 1C constant current charge ratio was 80.7%, and the capacity retention rate after 50 cycles of 1C was 95.0%. Using the Sansi powder compaction test equipment, the compaction density was 2.25g / cc at a pressure of 3T. It can be seen that the use of a solid solution type ferromanganese precursor with uniform distribution of non-iron manganese will significantly affect the structural stability of the material's charge and discharge cycle.

[0036] Comparative Example 2 The difference between this comparative example and Example 1 is that a non-pure phase manganese iron oxide containing some impure phases, namely (MnFe)2O3 and some Fe4O3 impure phases, is used as a precursor, and the remaining steps are the same as in Example 1.

[0037] The button cell was prepared using the lithium manganese iron phosphate positive electrode material prepared in this comparative example. The charging and discharging behavior of the button cell was tested and analyzed using the Blue Electric test system using the constant current-constant voltage charging (CC-CV) and constant current discharge (DC) test methods. The 1C discharge capacity was 140.3mAh / g, the 1C constant current charge ratio was 82.8%, and the capacity retention rate after 50 cycles of 1C was 97.2%. Using the Sansi powder compaction test equipment, the compaction density was 2.26g / cc at a pressure of 3T. This shows that non-pure phase manganese iron precursors will also affect the cyclic stability of the material.

[0038] Comparative Example 3 The difference between this comparative example and Example 1 is that the mass ratio of glucose to the iron manganese oxide precursor is 0.16:1, and lithium iron manganese phosphate is prepared by only one-step sintering, and the remaining steps are the same as those in Example 1.

[0039] The button cell was prepared using the lithium manganese iron phosphate positive electrode material prepared in this comparative example. The charging and discharging behaviors of the button cell were tested and analyzed using the Blue Electric test system using the constant current-constant voltage charging (CC-CV) and constant current discharging (DC) test methods. The 1C discharge capacity was 118.0mAh / g, the 1C constant current charge ratio was 66.5%, and the capacity retention rate after 50 cycles of 1C was 71.3%. It can be seen that the compaction density was 2.33g / cc at a pressure of 3T using the Sansi powder compaction test equipment. The use of one-step sintering will significantly affect the carbon coating effect, and the primary particles will adhere and fuse significantly. Li + The difficulty of embedding and de-embedding increases, and the rate performance and cycle performance deteriorate significantly.

[0040] Comparative Example 4 The difference between this comparative example and Example 1 is that only Ti is doped to obtain lithium manganese iron phosphate. The remaining steps are the same as those in Example 1.

[0041] The lithium iron manganese phosphate positive electrode material prepared in this comparative example was used to prepare a button battery. The charging and discharging behavior of the button battery was tested and analyzed by the Blue Electric test system using constant current-constant voltage charging (CC-CV) and constant current discharge (DC) test methods. The 1C discharge capacity was 132.7mAh / g, the 1C constant current charge ratio was 72.5%, and the capacity retention rate after 50 cycles of 1C was 88.9%. Using the Sansi powder compaction test equipment, the compaction density was 2.30g / cc at a pressure of 3T. It can be seen that the rate performance of the positive electrode material is still poor when a single element is doped, and composite doping technology is required to significantly improve the conductivity of the material through the synergistic effect of multiple elements.

[0042] The test results of the above embodiments and comparative examples are shown in Table 1, which shows that the positive electrode material prepared in the embodiment has a higher energy density and can meet the needs of high-rate applications; the polarization is smaller during the charging process, and the lithium ion insertion and extraction kinetics are better; it proves that the material has better structural stability and cycle life; the material particle morphology and particle size distribution are reasonable, and it is suitable for electrode processing technology.

[0043] Table 1 Experimental results of examples and comparative examples

Claims

1. A method for preparing a high-cycle and high-rate lithium manganese iron phosphate positive electrode material, characterized in that: The steps include: S1, preparing an iron-manganese solution using ferrous salt, divalent manganese salt and a first doping metal salt as raw materials, mixing with a precipitant, reacting and aging, filtering, washing and drying to obtain an iron-manganese solid solution intermediate; S2, sintering the iron-manganese solid solution intermediate obtained in step S1 in an oxidizing atmosphere to obtain a pure-phase iron-manganese oxide precursor having uniform distribution of manganese and iron at the atomic level; the total content of the first doping metal in the iron-manganese oxide precursor is 0-10000 mass ppm; S3, dissolving the iron manganese oxide precursor, the lithium source, the phosphorus source, the first carbon source, and the second doping metal salt in pure water, wet grinding to a target particle size, spraying, drying, and sintering to obtain a first lithium manganese iron phosphate; S4, dissolving the first lithium manganese iron phosphate, the second carbon source and the third doped metal salt in pure water in step S3, grinding to a target particle size by wet method, spraying, drying and sintering to obtain a second lithium manganese iron phosphate, i.e., a lithium manganese iron phosphate positive electrode material.

2. The method for preparing a high-cycle and high-rate lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step S1: The ferrous salt is one or more of ferrous sulfate, ferrous chloride, ferrous oxalate, ferrous nitrate, ferrous acetate and hydrates thereof; The divalent manganese salt is one or more of manganese sulfate, manganese chloride, manganese oxalate, manganese nitrate, manganese acetate and hydrates thereof; The precipitant is one or more of oxalic acid, ammonium oxalate, ammonium hydrogen oxalate, sodium oxalate, sodium hydrogen oxalate, potassium oxalate, potassium hydrogen oxalate, ammonium carbonate, ammonium hydrogen carbonate, sodium carbonate, sodium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, sodium hydroxide, potassium hydroxide, and ammonia water; The first doping metal salt is one or more of vanadium salt, magnesium salt, nickel salt, cobalt salt, titanium salt, copper salt, niobium salt, chromium salt, zinc salt, molybdenum salt and tungsten salt.

3. The method for preparing a high-cycle and high-rate lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step S1: The concentration of the ferrous ions is 0.15-1.98 mol / L, and the concentration of the divalent manganese ions is 0.22-1.35 mol / L; In terms of molar amount, the molar ratio of ferrous ion to divalent manganese ion is 1:9 to 9:1; The molar ratio of the amount of the precipitant added to Mn²⁺+Fe²⁺ is 1.0-2.5:

1.

4. The method for preparing a high-cycle and high-rate lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step S1: The reaction time is 0.1-5 hours and the temperature is 20-90° C.; the aging time is 0.5-20 hours; the drying temperature is 50-120° C. and the drying duration is 4-24 hours.

5. The method for preparing a high-cycle and high-rate lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step S2: the sintering temperature is 500-900° C., and the sintering time is 3-10 hours.

6. The method for preparing a high-cycle and high-rate lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step S3: The lithium source is one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate, lithium oxalate, and lithium acetate; The phosphorus source is one or more of phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and lithium phosphate; The first carbon source is one or more of glucose, sucrose, fructose, starch, PEG, PVP, cyclodextrin, and citric acid; The second doping metal salt is one or more of vanadium salt, magnesium salt, nickel salt, cobalt salt, titanium salt, copper salt, niobium salt, chromium salt, zinc salt, molybdenum salt and tungsten salt.

7. The method for preparing a high-cycle and high-rate lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step S3: The molar ratio of the lithium source, the iron source + manganese source of the iron manganese oxide precursor, the second doping metal salt, and the phosphorus source is 1.01-1.06:0.92-1.02:0-0.08:1; The mass ratio of the first carbon source to the iron manganese oxide precursor is 0.05-0.15:1; The target particle size of the wet grinding is D50 = 0.03-0.06 microns; During sintering, the sintering temperature is 350-680°C, the sintering time is 3-10 hours, and the heating rate is 0.5-5.0°C / min; The solid content in the reaction system is 30-50wt%.

8. The method for preparing a high-cycle and high-rate lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step S4: The third doping metal salt is one or more of vanadium salt, magnesium salt, nickel salt, cobalt salt, titanium salt, copper salt, niobium salt, chromium salt, zinc salt, molybdenum salt and tungsten salt; The mass ratio of the first lithium manganese iron phosphate to the third doping metal salt is 1:0-0.01; The feeding mass ratio of the second carbon source to the first lithium manganese iron phosphate is 0.05-0.15:1; The target particle size of the wet grinding is D50 = 0.2-0.6 microns; The solid content in the reaction system is 15-45wt%.

9. The method for preparing a high-cycle and high-rate lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step S4: During sintering, the sintering temperature is 650-800° C., the sintering time is 3-10 hours, and the heating rate is 0.5-5.0° C. / min.

10. The method for preparing a high-cycle and high-rate lithium iron manganese phosphate positive electrode material according to claim 9, characterized in that: In step S4: The second carbon source is one or more of glucose, sucrose, fructose, citric acid, phenolic resin, polyvinyl alcohol, polyethylene glycol, polyvinyl pyrrolidone, starch, carbon black, acetylene black, graphite, graphene, fullerene, and conductive carbon tube; The total amount of doped metal in the second lithium manganese iron phosphate is 5000-20000 mass ppm.

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