CN115676797B - Lithium iron manganese phosphate material, preparation method and application thereof - Google Patents
Lithium iron manganese phosphate material, preparation method and application thereof Download PDFInfo
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Abstract
The invention provides a lithium iron manganese phosphate material, a preparation method and application thereof. S1, grinding manganese powder and iron powder serving as raw material powder to obtain nano mixed metal powder; s2, performing ball milling treatment on the nano mixed metal powder, the lithium source and the phosphorus source for the first time to obtain a first ball grinding material; s3, performing secondary ball milling treatment on the primary ball grinding material and the fluxing agent to obtain a secondary ball milling material; and S4, sintering the secondary ball abrasive under oxidizing gas and reducing gas to obtain the lithium iron manganese phosphate material. The preparation method adopts one-step sintering, can reduce intermediate products, simplify the production flow and improve the production efficiency; in addition, the invention adopts manganese powder and iron powder as raw material powder, realizes the nanocrystallization of the mixed metal powder through grinding treatment, ensures that the mixed material is more uniform, and can further reduce the sintering treatment temperature by adding the fluxing agent.
Description
Technical Field
The invention belongs to the technical field of new electrochemical materials, and particularly relates to a lithium iron manganese phosphate material, a preparation method and application thereof.
Background
Lithium iron manganese phosphate (LiMn) (1-x) Fe x PO 4 ) The theoretical capacity of (C) is 170mAh/g as that of lithium iron phosphate, but it is relative to Li + The electrode potential of/Li is 4.1V, which is far higher than 3.4V of lithium iron phosphate, so that the lithium iron manganese phosphate has the potential of manufacturing a high-energy high-density power battery, and the theoretical energy density is 10% -20% higher than that of the lithium iron phosphate. Although lithium manganese phosphate has the characteristics of high voltage, high energy density and high safety performance, the olivine type crystal structure limits the migration of lithium ions, so that the lithium manganese phosphate has lower electronic conductivity and ionic conductivity, and the capacity exertion and the influence of the lithium manganese phosphate are influencedThe rate capability, the conductivity of the lithium iron manganese phosphate is 5-6 orders of magnitude lower than that of the lithium iron phosphate, and a material with good doping and coating conductivity is generally required. In the prior art, in order to improve the conductivity of the lithium iron manganese phosphate, the lithium iron manganese phosphate is generally modified by means of nanocrystallization, carbon coating, ion doping and the like, however, the modification means can reduce the compaction density of the lithium iron manganese phosphate and increase the specific surface area, so that the volume energy density and the processing performance of the material are affected.
The existing synthesis methods of the lithium iron manganese phosphate include a solid phase method, a liquid phase method, a hydrothermal method, a gel method and the like. The solid phase method utilizes an iron source, a manganese source, a lithium source and a phosphorus source to synthesize the lithium iron phosphate, the raw materials are insufficiently contacted in microcosmic, the mixing effect is limited, the synthesized product has uneven phases, wide particle size distribution range, poor product consistency, easy occurrence of impurities, longer synthesis period and larger energy consumption; the liquid phase method is to mix soluble manganese, iron and lithium source materials and then coprecipitate them to react, adjust pH with alkaline matter, dry them to obtain precursor materials, finally mix and sinter the precursor materials and phosphorus source to obtain lithium iron manganese phosphate materials. The hydrothermal method is to obtain lithium iron manganese phosphate through mixing reaction under a certain temperature in an aqueous environment, and has long synthesis period of the lithium iron manganese phosphate through hydrothermal synthesis, large sample particle size and poor electrochemical performance; the gel method is to mix soluble manganese, iron and lithium source materials, then add the glue solution, adjust the pH value by ammonia water or sodium hydroxide, evaporate the water in the glue solution, dry to form xerogel precursor, and get the final product after microwave treatment.
The invention patent CN110980682A discloses a preparation method of a lithium iron manganese phosphate material, which comprises the steps of performing coprecipitation reaction on manganese salt, ferric salt, oxalic acid or phosphoric acid solution in a hypergravity rotating bed, washing, filtering, mixing with water, adding a carbon source, and calcining at a high temperature to obtain the lithium iron manganese phosphate material, wherein the preparation method has the advantages of complex equipment, high control difficulty, more process detection and low production efficiency.
The invention patent CN114644328A discloses a preparation method of a lithium iron manganese phosphate material, which comprises the following steps: firstly, solid-phase mixing of a manganese source and/or an iron source to obtain a first mixture; then the first mixture is subjected to solid phase sintering at 300-1200 ℃ to obtain ferromanganese oxide (Mn) x Fe 1-x-y ) m O n The method comprises the steps of carrying out a first treatment on the surface of the Thereafter, manganese iron oxide (Mn x Fe 1-x-y ) m O n Solid phase mixing with a lithium source, a phosphorus source and optionally a manganese source and/or an iron source to obtain a second mixture; then the second mixture is subjected to solid phase sintering at 350-900 ℃ to obtain lithium manganese iron phosphate LiMn x Fe 1-x-y PO 4 However, the preparation method adopts a traditional two-stage sintering mode, wherein a precursor is formed by primary sintering, and then a material is formed by secondary sintering, and the traditional sintering method has the problems of uneven material mixing, complicated secondary sintering steps, large sintering temperature difference, large sintering temperature switching frequency and the like, so that the production efficiency is low, the primary qualification rate is reduced, and the problems of excessive manpower and material resources and increased cost are increased although the problems can be solved by enhancing the process control requirement and increasing the detection frequency.
The invention patent CN109761210A discloses a preparation method of a lithium iron manganese phosphate material, which comprises the steps of adding a manganese source, an iron source and a surfactant into an acidic hydrogen peroxide solution, transferring the solution into a high-temperature reaction kettle for reaction, and filtering and sintering the solution to obtain lithium iron manganese phosphate.
The invention patent CN105226273A discloses a preparation method of a lithium iron phosphate material, which adopts a sol-gel method to prepare a lithium iron phosphate sol and a lithium manganese phosphate sol, and then the lithium iron phosphate sol and the lithium manganese phosphate sol are calcined in an inert atmosphere to obtain the lithium iron phosphate material, however, the preparation process of the preparation method is complicated, two gels are required to be prepared respectively, experimental reagents are numerous, and the gelation process is difficult to control.
The invention patent CN106328942A discloses a preparation method of a lithium iron manganese phosphate anode material: mixing a lithium source, a manganese source, an iron source, a phosphorus source solution and a pore-forming agent polymer solution to obtain a spinning solution, carrying out electrostatic spinning on the spinning solution to obtain a lithium iron manganese phosphate precursor, and sintering to further obtain the lithium iron manganese phosphate anode material.
Disclosure of Invention
Aiming at the defects and the shortcomings existing in the prior art, the invention aims to provide a lithium iron manganese phosphate material, a preparation method and application thereof. Firstly, grinding manganese powder and iron powder serving as raw material powder to obtain nano mixed metal powder with the D50 particle size less than 500nm, wherein the grinding treatment can fully mix the manganese powder and the iron powder, so that the problem of non-uniform solid phase mixing in the traditional method is solved; then carrying out ball milling treatment on the mixed metal powder, the lithium source and the phosphorus source for the first time to obtain a uniformly mixed primary ball grinding material with smaller granularity, thereby ensuring the distribution uniformity of each element of phosphorus, manganese, iron and lithium in the primary ball grinding material; then, carrying out secondary ball milling treatment on the primary ball grinding material and the fluxing agent to obtain a secondary ball milling material, wherein the addition of the fluxing agent can reduce the melting point of the metal element, so that the temperature of the subsequent sintering treatment is reduced; finally, sintering the secondary ball abrasive under oxidizing gas and reducing gas, and controlling the valence state of Fe by introducing the volume ratio of the oxidizing gas to the reducing gas, thereby controlling the structure of the lithium manganese iron phosphate material; the preparation method adopts one-step sintering, can reduce intermediate products, simplify the production flow and improve the production efficiency; in addition, the invention adopts manganese powder and iron powder as raw material powder, realizes the nanocrystallization of the mixed metal powder through grinding treatment, ensures that the mixed material is more uniform, and can further reduce the sintering treatment temperature by adding the fluxing agent. The primary particles of the lithium iron manganese phosphate material prepared by the method are uniform, have larger porosity and larger compaction density, and can greatly improve gram capacity and energy density of the material.
The invention provides a preparation method of a lithium iron manganese phosphate material, which adopts the following technical scheme:
a preparation method of a lithium iron manganese phosphate material comprises the following steps:
s1, grinding manganese powder and iron powder serving as raw material powder to obtain nano mixed metal powder;
s2, performing ball milling treatment on the nano mixed metal powder, the lithium source and the phosphorus source for the first time to obtain a first ball grinding material;
s3, performing secondary ball milling treatment on the primary ball grinding material and the fluxing agent to obtain a secondary ball milling material;
and S4, sintering the secondary ball abrasive under oxidizing gas and reducing gas to obtain the lithium iron manganese phosphate material.
According to the invention, the primary ball grinding material with smaller granularity, uniform mixing and uniform distribution of each element is obtained through primary ball milling treatment; then, adding a fluxing agent for secondary ball milling treatment, so that the fluxing agent can be better mixed with the primary ball grinding material, and the melting difference of each component in the sintering process can be reduced while the subsequent sintering treatment temperature is reduced; the effect difference of each component caused by the granularity difference can be reduced through two ball milling treatments; in addition, in the industrial continuous production, the two ball milling processes can be simultaneously carried out in different ball mills, so that the mixing efficiency is not affected; if the fluxing agent is directly added into the mixed metal powder, the lithium source and the phosphorus source for one-time ball milling treatment, the mixing time is prolonged, the efficiency is low, and the further prolonged mixing time can possibly cause the structural damage of the raw materials of each component.
In addition, the sintering treatment is carried out under the oxidizing gas and the reducing gas, and the valence state of Fe can be controlled by controlling the volume ratio of the two gases, so that the structure of the lithium iron manganese phosphate material is controlled; if the sintering process is performed under a conventional shielding gas such as nitrogen, the nitrogen reacts with the flux (lithium fluoride, etc.), while other common inert gases such as helium, argon, etc. have higher cost, and the oxidizing gas and the reducing gas adopted in the invention have low cost. Compared with the traditional solid phase sintering method, the method adopts the two-step sintering under the condition of inert gas, the method adopts the manganese powder and the iron powder with the close melting points as the raw material powder, and the raw material powder is ground to obtain the nano mixed metal powder, so that the melting point is reduced while the mixing uniformity is ensured, and the reaction of the raw materials of all components is further ensured under the lower temperature by the action of a fluxing agent.
In the above preparation method, as a preferred embodiment, in the step S1, the D50 particle diameter of the manganese powder is < 10 μm; the D50 particle size of the iron powder is less than 10 mu m, and the molar ratio of the iron element in the iron powder to the manganese element in the manganese powder is 0.8-0.5:0.5-0.2 (such as 0.75:0.25, 0.7:0.3, 0.65:0.35, 0.6:0.4 and 0.55:0.55).
The molar ratio of the iron element in the iron powder to the manganese element in the manganese powder is limited to 0.8-0.5:0.5-0.2, the lithium iron manganese phosphate material prepared in the range has better electrical property, and if the content of manganese element is too high, the electrical property of the material is reduced.
In the above preparation method, as a preferred embodiment, in the step S1, the D50 particle diameter of the nano-sized mixed metal powder is < 500nm; preferably, when the particle size of the nano mixed metal powder is detected by a particle size analyzer and the components of the nano mixed metal powder are detected by an atomic analysis spectrometer, respectively taking the upper layer powder, the middle layer powder and the lower layer powder of the nano mixed metal powder as a group, and respectively taking three samples for detection; preferably, the D50 particle size of the samples in each group is less than 500nm, the purity is more than or equal to 99.5%, and the nano mixed metal powder is qualified and is used in the primary ball milling treatment.
In the above preparation method, as a preferred embodiment, in the step S1, the grinding treatment is performed in a grinding tank using zirconium beads as a grinding medium at a grinding speed of 1200 to 1500rpm (e.g., 1300rpm, 1400rpm, 1500 rpm) for 2 to 6 hours (e.g., 3 hours, 4 hours, 5 hours).
According to the invention, the manganese powder with the D50 particle size smaller than 10 mu m is adopted, the iron powder with the D50 particle size smaller than 10 mu m is used as the raw material powder, the nanocrystallization of the mixed metal powder is realized through grinding treatment, and the sufficient mixing of the manganese powder and the iron powder is ensured, so that the problem of nonuniform solid phase mixing in the traditional method is solved; the particle size detection and the component analysis are carried out on the upper layer, the middle layer and the lower layer of the nano mixed metal powder, so that the D50 particle size of the mixed metal powder is less than 500nm, the purity is more than or equal to 99.5%, the problem of uneven mixing caused by particle size difference and component difference when two different metal powders are mixed is further solved, and a foundation is provided for obtaining the ball milling material with uniform distribution of each element subsequently; if the D50 particle size of the mixed metal powder is too large, even mixing of the manganese powder and the iron powder under the nano-size cannot be realized, so that the raw materials are not fully contacted in a microcosmic manner, the mixing effect is limited, and finally, the synthesized lithium iron manganese phosphate material is uneven in phase and wide in particle size distribution range.
In the above preparation method, as a preferred embodiment, in the step S2, the lithium source is selected from one or more of lithium powder, lithium carbonate, lithium hydroxide; the phosphorus source is one or more of white phosphorus, solid phosphoric acid, ferric phosphate and manganese phosphate;
preferably, the molar ratio of the total amount of manganese element and iron element in the mixed metal powder to the lithium element in the lithium source and the phosphorus element in the phosphorus source is 0.5-2:0.5-2:0.5-2 (e.g., 0.8:0.8:0.8, 1:1:1, 1.5:1.5:1.5).
According to the preparation method, the lithium source and the phosphorus source are both solid phases, and the solid phases are mixed in the primary ball milling treatment and the secondary ball milling treatment, so that a precursor is not required to be prepared in a liquid phase environment and then sintered, and the preparation method is simpler and has higher synthesis efficiency in terms of the preparation process; in addition, the invention adopts high-melting-point phosphorus sources such as ferric phosphate, so that the difference of the reaction temperature of the phosphorus sources and metal Fe and metal Mn can be reduced, the consistency of the melting reaction of each element in the sintering treatment is ensured, and the consistency of products is further improved; wherein, the solid phosphoric acid is prepared by mixing phosphoric acid and phosphorus pentoxide, adding diatomite, and is powdery or massive solid at normal temperature and pressure.
In the above preparation method, as a preferred embodiment, in the step S2, the rotational speed of the one ball milling process is 400-800rpm (e.g., 500rpm, 600rpm, 700 rpm), and the one ball milling time is 4-20 hours (e.g., 5 hours, 8 hours, 10 hours, 15 hours, 18 hours).
In the above preparation method, as a preferred embodiment, in the step S3, the flux is one or more selected from lithium fluoride, aluminum fluoride, and magnesium fluoride; the fluxing agent is preferably lithium fluoride; preferably, the mass of the fluxing agent is 0.05% -0.5% (e.g., 0.06%, 0.08%, 0.1%, 0.2%, 0.3%, 0.4%) of the mass of the primary ball abrasive.
The addition of the fluxing agent can realize synergistic melting, because certain difference exists in the melting points of the raw materials of all the components in the sintering process, although the melting temperature of the raw materials can be greatly reduced by grinding treatment to realize the nanocrystallization of the mixed metal powder, certain difference exists, in order to avoid the condition that the raw materials of some components are over-burned or insufficiently sintered in the sintering process, the fluxing agent is added, so that the raw materials of all the components can fully react in a certain temperature range, and meanwhile, in order to consider that impurity ions are not introduced, lithium fluoride is preferred, wherein F element can improve the electronic conductivity of the lithium iron manganese phosphate material, and Li can be embedded into crystal lattices, so that the addition amount of lithium powder can be reduced properly, and the cost is further reduced. If the mass of the fluxing agent is too little, the melting point of the mixed metal powder is higher, the melting temperature is reduced less at the same sintering temperature, so that the metal is not completely melted, the metal reaction is not complete, and finally, the proportion of the composite material is abnormal, and the electrical property is reduced; if the mass of the fluxing agent is excessively added, under the same sintering temperature, metal is melted in advance, even other component raw materials are melted in advance, overburning is caused, physical and chemical data of the finally prepared lithium iron manganese phosphate material is poor, electrical performance is poor, cost is increased due to excessive addition, impurities of the material are increased, and pretreatment difficulty is increased.
In the above preparation method, as a preferred embodiment, in the step S3, the rotational speed of the secondary ball milling treatment is 400-800rpm (e.g., 500rpm, 600rpm, 700 rpm), and the secondary ball milling time is 20-120min (e.g., 30min, 50min, 80min, 90min, 100 min).
In the above preparation method, as a preferred embodiment, in the step S4, the oxidizing gas is selected from one of oxygen, nitrous oxide, and sulfur trioxide; the reducing gas is selected from one of hydrogen, carbon monoxide and sulfur monoxide; the volume ratio of the oxidizing gas to the reducing gas is 8-6:2-4 (e.g., 7.5:2.5, 7.5:3, 7.5:3.5, 7:3.5, 7:3, 7:2.5).
According to the invention, the valence state of Fe is controlled by limiting the volume ratio of oxidizing gas to reducing gas to 8-6:2-4, so that the structure of the lithium iron manganese phosphate material is controlled; if the ratio of the oxidizing gas is too large, the Fe element is oxidized into trivalent in the sintering process, the valence state of the Fe element is increased, the purity of the lithium iron manganese phosphate is further reduced, and meanwhile, the structural stability of the material is reduced, so that the electrical performance is influenced; if the reducing gas is too much, this leads to increased costs and corresponding safety problems.
In the above preparation method, as a preferred embodiment, in the step S4, the sintering treatment is performed in a sintering furnace at a sintering temperature of 500 to 1000 ℃ (e.g., 600 ℃, 700 ℃, 800 ℃, 900 ℃, 950 ℃) for a sintering time of 8 to 24 hours (e.g., 10 hours, 12 hours, 15 hours, 18 hours, 20 hours).
The sintering temperature of the sintering treatment is limited to 500-1000 ℃, and the sintering treatment is favorable for obtaining the lithium iron manganese phosphate material with sufficient sintering and consistent phase within the temperature range; if the sintering temperature is too low, the sintering is insufficient, the crystal form of the material is poor, and meanwhile, the problem of uneven distribution of elements in the material can be caused; if the sintering temperature is too high, the crystal growth of the material is too large, the tap density is too high, and the conductivity is reduced.
In a second aspect, the invention provides a lithium iron manganese phosphate material prepared byThe lithium manganese iron phosphate material is prepared by the preparation method, and has the chemical formula: liMn (1-x) Fe x PO 4 Wherein 0.5.ltoreq.x.ltoreq.0.8 (e.g. x=0.55, x=0.6, x=0.7, x=0.75).
In the above lithium iron manganese phosphate material, as a preferred embodiment, the microstructure of the lithium iron manganese phosphate material is a secondary microparticle composed of primary particles; the primary particles have diameters of 60-200nm (such as 80nm, 100nm, 120nm, 150nm, 180 nm), the secondary particles have diameters of 5-20 μm (such as 8 μm, 10 μm, 12 μm, 15 μm, 18 μm), and specific surface areas of 9.0-22.0g/m 2 (e.g. 10g/m 2 、12g/m 2 、15g/m 2 、17g/m 2 、19g/m 2 ) The tap density is more than or equal to 1.4g/m 3 The compaction density is more than or equal to 2.6g/m 3 The porosity is more than 84 percent.
The primary particles of the lithium iron phosphate material obtained by the preparation method are uniform, have the advantages of larger porosity and larger compaction density, and can improve the gram capacity and the energy density of the material. In the sintering process, the molten component raw materials form primary particles, the primary particles are fused and grown to form secondary particles, and the secondary particles gradually grow up along with the sintering process to obtain the lithium iron manganese phosphate material with the microscopic morphology of the secondary particles consisting of the primary particles.
The third aspect of the invention provides an application of the lithium iron manganese phosphate material or the lithium iron manganese phosphate material prepared by the preparation method in a lithium ion battery.
Compared with the prior art, the invention has the following advantages:
(1) The preparation method adopts one-step sintering, can reduce intermediate products, simplify the production flow and improve the production efficiency;
(2) According to the invention, manganese powder and iron powder are used as raw material powder, and the nanocrystallization of the mixed metal powder is realized through grinding treatment, so that the mixed material is more uniform, and the problem of nonuniform solid-phase mixed material in the prior art is solved.
(3) The addition of the fluxing agent in the invention can reduce the sintering temperature, thereby ensuring that the raw materials of each component can fully react within a certain temperature range, and meanwhile, in order to avoid introducing impurity ions, the fluxing agent is preferably lithium fluoride, wherein the F element can improve the electronic conductivity of the lithium iron manganese phosphate material, and Li can be intercalated into crystal lattices, so that the addition amount of lithium powder can be reduced properly, and the cost is further reduced.
(4) The invention limits the volume ratio of the introduced oxidizing gas and the reducing gas in the sintering treatment, and can control the oxidation degree of Fe, thereby ensuring the structure of the lithium iron manganese phosphate material.
Drawings
FIG. 1 is an SEM image of a lithium iron manganese phosphate material prepared according to example 1 of the present invention;
FIG. 2 is an SEM image of a lithium iron manganese phosphate material prepared according to example 2 of the present invention;
FIG. 3 is an SEM image of a lithium iron manganese phosphate material prepared according to example 3 of the present invention;
FIG. 4 is an SEM image of a lithium iron manganese phosphate material prepared according to example 4 of the present invention;
FIG. 5 is an SEM image of a lithium iron manganese phosphate material prepared according to example 5 of the present invention;
FIG. 6 is an SEM image of a lithium iron manganese phosphate material prepared according to example 6 of the present invention;
FIG. 7 is an SEM image of a lithium iron manganese phosphate material prepared according to comparative example 1 of the present invention;
FIG. 8 is an SEM image of a lithium iron manganese phosphate material prepared according to comparative example 2 of the present invention;
FIG. 9 is an SEM image of a lithium iron manganese phosphate material prepared according to comparative example 3 of the present invention;
FIG. 10 is an SEM image of a lithium iron manganese phosphate material prepared according to comparative example 4 of the present invention;
FIG. 11 is an SEM image of a lithium iron manganese phosphate material prepared according to comparative example 5 of the present invention;
FIG. 12 is an SEM image of a lithium iron manganese phosphate material prepared according to comparative example 6 of the present invention;
FIG. 13 is an SEM image of a lithium iron manganese phosphate material prepared according to comparative example 7 of the present invention;
FIG. 14 is an SEM image of a lithium iron manganese phosphate material prepared according to comparative example 8 of the present invention;
fig. 15 is an XRD pattern of the lithium iron manganese phosphate material prepared in example 1 of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions in the embodiments of the present invention will be clearly and completely described in the following in conjunction with the embodiments of the present invention. It will be apparent to those skilled in the art that the examples are merely to aid in understanding the invention and are not to be construed as a specific limitation thereof. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The examples of the present invention are implemented on the premise of the technical scheme of the present invention, and detailed implementation modes and processes are given, but the protection scope of the present invention is not limited to the following examples, in which the process parameters of specific conditions are not noted, and generally according to conventional conditions.
The endpoints of the ranges and any values disclosed in the present invention are not limited to the precise range or value, and the range or value should be understood to include values close to the range or value. For numerical ranges, one or more new numerical ranges may be obtained in combination with each other between the endpoints of each range, between the endpoint of each range and the individual point value, and between the individual point values, and are to be considered as specifically disclosed in the present invention.
In the present invention, all values relating to the amounts of the components are "parts by weight" throughout unless specified and/or indicated otherwise. The process parameters for the specific conditions not noted in the examples below are generally as usual. The starting materials described in the examples below are all commercially available from the public.
The present invention will be described in further detail with reference to specific examples.
Example 1 a method for preparing a lithium iron manganese phosphate material, comprising:
(1) Adding iron powder with the D50 granularity less than 10 mu m and manganese powder with the D50 granularity less than 10 mu m into a grinding tank according to the molar ratio of iron element to manganese element of 0.8:0.2, and grinding, wherein a grinding medium is zirconium balls with the size of 0.5mm, the grinding rotating speed is 1500rpm, the grinding time is 4 hours, so as to obtain mixed metal powder, respectively taking upper layer powder, middle layer powder and lower layer powder of the mixed metal powder as a group, respectively taking three samples in each group, detecting the granularity of the mixed metal powder by adopting a granularity meter and detecting the components of the mixed metal powder by adopting an atomic analysis spectrometer, and obtaining the mixed metal powder with the D50 granularity less than 500nm and the purity more than or equal to 99.5 percent in each sample;
(2) Mixing the mixed metal powder obtained in the step (1) with lithium powder and ferric phosphate according to the mole ratio of iron element and manganese element in the mixed metal powder, lithium element in the lithium powder and phosphorus element in the ferric phosphate of 1:1:1, adding the mixture into a ball milling tank for ball milling treatment for 12 hours at the ball milling rotating speed of 600rpm to obtain a primary ball grinding material;
(3) Mixing lithium fluoride with the primary ball grinding material obtained in the step (2) according to the mass ratio of 0.2%:1, adding the mixture into a ball milling tank for secondary ball milling treatment, wherein the ball milling rotating speed is 600rpm, and the time is 30min, so as to obtain a secondary ball milling material;
(4) Sintering the secondary ball abrasive in a sintering furnace under the gas atmosphere with the volume ratio of oxygen to hydrogen of 8:2, wherein the sintering temperature is 760 ℃ and the sintering time is 16 hours, so as to obtain the lithium iron manganese phosphate material (the SEM image of the lithium iron manganese phosphate material prepared in the embodiment 1 of the invention is shown in figure 1, primary particles of the obtained lithium iron manganese phosphate material can be uniformly distributed in figure 1, and the XRD image of the lithium iron manganese phosphate material prepared in the embodiment 1 of the invention is shown in figure 15).
Example 2
Example 2 is different from example 1 in that the molar ratio of iron element in the iron powder to manganese element in the manganese powder is 0.7:0.3, and the rest is the same as example 1, (fig. 2 is an SEM image of the lithium iron manganese phosphate material prepared in example 2 of the present invention).
Example 3
Example 3 is different from example 1 in that the molar ratio of iron element in the iron powder to manganese element in the manganese powder is 0.6:0.4, and the rest is the same as example 1, (fig. 3 is an SEM image of the lithium iron manganese phosphate material prepared in example 3 of the present invention).
Example 4
Example 4 differs from example 1 in that the molar ratio of iron element in the iron powder to manganese element in the manganese powder is 0.5:0.5, and the rest is the same as example 1, (fig. 4 is an SEM image of the lithium iron manganese phosphate material prepared in example 4 of the present invention).
Example 5
Example 5 differs from example 4 in that the sintering temperature of the sintering treatment in step (4) is 500 ℃, and the rest is the same as example 4 (fig. 5 is an SEM image of the lithium iron manganese phosphate material prepared in example 5 of the present invention).
Example 6
Example 6 differs from example 4 in that the sintering temperature of the sintering treatment in step (4) is 1000 ℃, and the rest is the same as example 4 (fig. 6 is an SEM image of the lithium iron manganese phosphate material prepared in example 6 of the present invention).
Comparative example 1 a method for preparing a lithium iron manganese phosphate material, comprising:
(1) Salt solution preparation: firstly, preparing a metal mixed salt solution of manganese chloride and ferrous chloride with the total concentration of metal ions of 0.06mol/L according to the mole ratio of iron element to manganese element of 0.5:0.5; then preparing ammonium dihydrogen phosphate solution with the concentration of 0.06 mol/L; then preparing ammonia water with the concentration of 10.0 mol/L;
(2) Preparing a base solution: adding ammonia water with the concentration of 10.0mol/L into the monoammonium phosphate solution, and regulating the pH value to 9 to obtain a base solution;
(3) Precipitation reaction: under the nitrogen atmosphere, the metal mixed salt solution is added into a reaction kettle containing 20L of base solution in parallel flow mode at 3L/min, the ammonium dihydrogen phosphate solution at 3L/min and the ammonia water at 20mL/min, the temperature in the reaction kettle is controlled to be 20 ℃, the pH value is controlled to be 8.5, and the stirring rotating speed is 350r/min; stopping feeding when the particle size of the material D50 in the kettle reaches 15 mu m, and performing solid-liquid separation to obtain a solid-phase precursor;
(4) Crushing and sintering: pulverizing solid phase precursor into powder with particle diameter of 2-5 μm; preparing a mixed solution of 0.8mol/L according to the molar ratio of Fe element and Mn element in a solid-phase precursor to lithium element in lithium hydroxide to carbon element in glucose of 0.5:0.5:1.1:0.3, performing spray pyrolysis (the spray pyrolysis condition is that the reaction temperature is 580 ℃, the gas carrying capacity is 80L/h) to obtain a solid, calcining the solid at 850 ℃ for 14h, and naturally cooling to room temperature to obtain a lithium iron manganese phosphate material (an SEM image of the lithium iron manganese phosphate material prepared in comparative example 1 in FIG. 7).
Comparative example 2a method of preparing a lithium iron manganese phosphate material, comprising:
(1) Solid phase precursor: mixing manganese sulfate and ferrous sulfate according to the molar ratio of manganese element in manganese sulfate to iron element in ferrous sulfate of 0.5:0.5 to obtain a mixture, and sintering the mixture at 920 ℃ for 12 hours to obtain a solid-phase precursor;
(2) Lithium iron manganese phosphate material: mixing the solid-phase precursor in the step (1) with lithium powder and ferric phosphate according to the molar ratio of the total amount of iron element and manganese element in the solid-phase precursor material to the molar ratio of lithium element in the lithium powder to the molar ratio of iron element in the ferric phosphate of 1:1:1 to obtain a mixture, and sintering the mixture at 800 ℃ for 16 hours to obtain a lithium manganese iron phosphate material (FIG. 8 is an SEM image of the lithium manganese iron phosphate material prepared in the comparative example 2).
Comparative example 3a method of preparing a lithium iron manganese phosphate material, comprising:
(1) Hydrothermal synthesis: 8.35g of lithium hydroxide was added to 50mL of deionized water under stirring, followed by 6mL of phosphoric acid (75 wt% concentration), 10.04g of ferrous sulfate, 6.32g of manganese sulfate, and 0.2g of ascorbic acid. Heating to 180 ℃ for hydrothermal reaction for 14min in a stirring state for 5min to obtain LiFe 0.5 Mn 0.5 PO 4 Solid solutions.
(2) Carbon coating: washing the synthesized solid solution with water; then, glucose and the lithium iron manganese phosphate solid solution are uniformly mixed according to the mass ratio of 1:4, and are put into a tube furnace for heat treatment, wherein the temperature of the heat treatment is 700 ℃, and the heat preservation time is 4 hours, so that the carbon-coated lithium iron manganese phosphate material (an SEM image of the lithium iron manganese phosphate material prepared in comparative example 3 in the invention) is finally obtained.
Comparative example 4 a method of preparing a lithium iron manganese phosphate material, comprising:
(1) Synthesizing lithium manganese phosphate sol: weighing 0.15mol of manganese nitrate, 0.15mol of lithium hydroxide, 0.15mol of diammonium hydrogen phosphate and 0.3mol of citric acid, adding deionized water to prepare 5mol/L,8mol/L,8mol/L and 8mol/L solutions respectively, mixing the manganese nitrate solution and the citric acid solution to form a first suspension, and adding the diammonium hydrogen phosphate into the first suspension to form a second suspension; finally, adding the lithium hydroxide solution into the second suspension to form a third suspension, dropwise adding ammonia water into the three suspensions to adjust the pH value to 3, and uniformly stirring to obtain transparent lithium manganese phosphate sol;
(2) And (3) synthesizing lithium iron manganese phosphate sol: weighing 0.15mol of ferric nitrate, 0.15mol of lithium hydroxide, 0.15mol of diammonium hydrogen phosphate and 0.3mol of citric acid, adding deionized water to prepare solutions of 1.22mol/L, 1.22mol/L and 1.22mol/L respectively, adding the diammonium hydrogen phosphate into the ferric nitrate solution to form a fourth suspension, and adding the fourth suspension into the citric acid solution to form a fifth suspension; adding lithium hydroxide solution into the fifth suspension to form a sixth suspension, adding ammonia water to adjust the pH value to 4, and uniformly stirring to form transparent lithium iron manganese phosphate sol;
(3) High-temperature calcination: mixing the two sols completely, drying the mixture of the lithium iron phosphate sol and the lithium manganese phosphate sol by spray drying equipment, and calcining for 8 hours at 750 ℃ under nitrogen atmosphere to obtain LiMn 0.5 Fe 0.5 PO 4 Composite positive electrode material/C (fig. 10 is an SEM image of the lithium iron manganese phosphate material prepared in comparative example 4 of the present invention).
Comparative example 5
Comparative example 5 differs from example 4 in that in step (1), iron powder having a D50 particle diameter of < 10 μm and manganese powder having a D50 particle diameter of < 10 μm are physically mixed (directly mixed) in a molar ratio of iron element to manganese element of 5:5 to obtain a mixed metal powder; the remainder was the same as in example 4 (fig. 11 is an SEM image of the lithium iron manganese phosphate material prepared in comparative example 5 of the present invention).
Comparative example 6
Comparative example 6 differs from example 4 in that step (3) was not performed, and the rest was the same as example 4 (fig. 12 is an SEM image of the lithium iron manganese phosphate material prepared in comparative example 6 of the present invention).
Comparative example 7
Comparative example 7 is different from example 4 in that the sintering temperature of the sintering treatment in step (4) is 400 deg.c, and the rest is the same as example 4 (fig. 13 is an SEM image of the lithium iron manganese phosphate material prepared in comparative example 7 of the present invention).
Comparative example 8
Comparative example 8 differs from example 4 in that the sintering temperature of the sintering treatment in step (4) was 1200 deg.c, and the rest was the same as example 4 (fig. 14 is an SEM image of the lithium iron manganese phosphate material prepared in comparative example 8 of the present invention).
Performance testing
The specific surface area, tap density and compaction density of the lithium iron phosphate material prepared in the embodiment 1 and the comparative examples 1 to 4 are detected, the porosity of the material is detected by a nitrogen adsorption and desorption method, and the detection results are shown in table 1;
the materials prepared in examples 1 to 6 and comparative examples 1 to 8 of the present invention were used as active materials, the binder was PVDF, the conductive agent was SP, and the active materials: and (2) a binder: the mass ratio of the conductive agent is 96:2:2, dissolving in CMC solvent to prepare slurry, uniformly coating the slurry on metal aluminum foil, vacuum drying, and finally cutting into round pole pieces with the diameter of 14mm by using a punch as a working electrode; in a purification glove box filled with Ar (the O2 content is less than 0.1ppm and the H2O content is less than 0.1 ppm), a metal lithium sheet is used as a counter electrode, a Celgard2400 porous propylene film is used as a diaphragm, an electrolyte is 1mol/L lithium hexafluorophosphate (LiPF 6) solution, and a solvent is Ethylene Carbonate (EC): ethyl carbonate (DMC) =1: 1, preparing a button cell (model CR 2032) according to a certain assembly process, and standing for 24 hours after the button cell is completed to fully infiltrate electrolyte and electrode materials; under the room temperature condition (25 ℃ plus or minus 1), the test condition is that the voltage is 2.5V-3.7V, the test current density is 150mAh/g, and the test results are shown in Table 2;
TABLE 1
Table 1 shows the physical and chemical properties of the lithium iron manganese phosphate materials prepared in example 1 and comparative examples 1 to 4, respectively, and it is clear from the above that the materials prepared by the preparation method of the present invention have high porosity, which is favorable for the exertion of electrical properties.
Table 2 shows the capacity and cycle performance at 0.1C magnification (0.1C is the nominal capacity at 0.1 times, i.e. working electrode load mass 0.1 current density), respectively.
TABLE 2
Note that: the median voltage is the average discharge voltage, that is, the average voltage of the battery obtained by a discharge curve during the discharge process, and the higher the median voltage is, the higher the energy density is.
As can be seen from Table 2, the lithium iron manganese phosphate materials prepared by the preparation methods of the present invention (examples 1 to 6) have higher capacity and better cycle performance than those of comparative examples 1 to 4.
The foregoing description of the preferred embodiments of the invention is not intended to limit the invention to the particular embodiments disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
Claims (9)
1. The preparation method of the lithium iron manganese phosphate material is characterized by comprising the following steps of:
s1, grinding manganese powder and iron powder serving as raw material powder to obtain nano mixed metal powder;
s2, performing ball milling treatment on the nano mixed metal powder, the lithium source and the phosphorus source for the first time to obtain a first ball grinding material;
s3, performing secondary ball milling treatment on the primary ball grinding material and the fluxing agent to obtain a secondary ball milling material;
s4, sintering the secondary ball abrasive under oxidizing gas and reducing gas to obtain a lithium iron manganese phosphate material;
in the step S3, the flux is lithium fluoride;
in the step S4, the oxidizing gas is selected from one of oxygen, nitrous oxide, and sulfur trioxide; the reducing gas is selected from one of hydrogen, carbon monoxide and sulfur monoxide; the volume ratio of the oxidizing gas to the reducing gas is 8-6:2-4; the sintering temperature is 500-1000 ℃.
2. The method according to claim 1, wherein in the step S1, the D50 particle size of the manganese powder is < 10 μm; the D50 particle size of the iron powder is less than 10 mu m, and the molar ratio of the iron element in the iron powder to the manganese element in the manganese powder is 0.8-0.5:0.5-0.2.
3. The preparation method according to claim 1 or 2, characterized in that in the step S1, the D50 particle size of the nanocrystallized mixed metal powder is < 500nm; and/or, when the particle size of the nano mixed metal powder is detected by adopting a particle size meter and the components of the nano mixed metal powder are detected by adopting an atomic analysis spectrometer, respectively taking the upper layer, the middle layer and the lower layer of powder of the nano mixed metal powder as a group, and respectively taking three samples for detection; the D50 particle size of the samples in each group is less than 500nm, the purity is more than or equal to 99.5%, and the nano mixed metal powder is qualified and is used in the primary ball milling treatment.
4. The method according to claim 3, wherein in the step S1, the grinding treatment is performed in a grinding tank using zirconium beads as a grinding medium at a grinding speed of 1200 to 1500rpm for a grinding time of 2 to 6 hours.
5. The method according to claim 1, wherein in the step S2, the lithium source is selected from one or more of lithium powder, lithium carbonate, lithium hydroxide; the phosphorus source is one or more of white phosphorus, solid phosphoric acid, ferric phosphate and manganese phosphate; the molar ratio of the total amount of manganese element and iron element in the mixed metal powder to the lithium element in the lithium source and the phosphorus element in the phosphorus source is 0.5-2:0.5-2:0.5-2; and/or the rotating speed of the primary ball milling treatment is 400-800rpm, and the primary ball milling time is 4-20h.
6. The method according to claim 1, wherein in the step S3, the mass of the flux is 0.05% -0.5% of the mass of the primary ball abrasive; and/or the rotating speed of the secondary ball milling treatment is 400-800rpm, and the secondary ball milling time is 20-120min.
7. The method according to claim 1, wherein in the step S4, the sintering treatment is performed in a sintering furnace for 8 to 24 hours.
8. A lithium iron manganese phosphate material prepared by the preparation method according to any one of claims 1 to 7, wherein the lithium iron manganese phosphate material has a chemical formula: liMn (1-x) Fe x PO 4 Wherein x is more than or equal to 0.5 and less than or equal to 0.8; the microcosmic appearance of the lithium iron manganese phosphate material is secondary particles composed of primary particles; the primary particles have a diameter of 60-200nm, the secondary particles have a diameter of 5-20 μm, and a specific surface area of 9.0-22.0g/m 2 The tap density is more than or equal to 1.4g/m 3 The compaction density is more than or equal to 2.6g/m 3 The porosity is more than 84 percent.
9. Use of a lithium iron manganese phosphate material prepared by the preparation method according to any one of claims 1-7 in a lithium ion battery.
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