CN115799468A - Iron-site-doped lithium manganese iron phosphate composite material, preparation method and secondary battery - Google Patents

Abstract

The application belongs to the technical field of battery materials, and particularly relates to an iron-site-doped lithium manganese iron phosphate composite material, a preparation method thereof, and a secondary battery. The iron site-doped lithium manganese iron phosphate composite material comprises a core and a carbon shell layer coated on the outer surface of the core, wherein the core comprises LiMn (chemical general formula) _0.6 Fe _{0.4‑x‑y‑z‑ w} Mg _x V _y Al _z A _w PO ₄ Wherein A comprises at least one of Ca, ba, ti and Zn; the value ranges of x, y and z are respectively and independently 0-0.05, and the value ranges of x, y and z are not 0,0 and not more than 0.05,0 and not more than x + y + z + w and not more than 0.15 at the same time. By doping iron sites in the lithium manganese iron phosphate materialThe mixed multiple metal ions make the catalyst have better rate performance and cycle stability. The carbon shell layer not only improves the conductivity of the composite material, but also improves the compaction density and specific capacity of the iron-site-doped lithium manganese iron phosphate composite material.

Description

Iron-site-doped lithium manganese iron phosphate composite material, preparation method and secondary battery

Technical Field

The application belongs to the technical field of battery materials, and particularly relates to an iron-doped lithium iron manganese phosphate composite material, a preparation method thereof and a secondary battery.

Background

With the rapid rise of the price of lithium battery raw materials and the entering of the new energy automobile subsidy policy in the period of grade withdrawal, the low-cost advantage of lithium iron phosphate is further amplified, and the output of the anti-super ternary material becomes the hottest lithium battery anode material. However, the specific capacity development of the current lithium iron phosphate material is close to the theoretical limit value, the energy density is difficult to further improve, and the search for a new cathode material is inevitable. Olivine-type LiMnPO ₄ Theoretical specific capacity and LiFePO ₄ The voltage platform can reach about 3.8V-4.1V and the theoretical energy density can be comparable to LiFePO ₄ 10-20% higher than the original product, and has good low-temperature performance. But is larger than LiFePO ₄ The lower conductivity seriously affects LiMnPO ₄ Discharge capacity and rate capability. Meanwhile, the dissolution of Mn in the circulation process enables the capacity retention rate of the battery to be rapidly reduced, and the LiMnPO is limited ₄ The development of (1).

Researches find that the working voltage of the lithium iron manganese phosphate solid solution obtained by combining the lithium iron manganese phosphate solid solution and the electrolyte is between 3.5 and 4.1V, the lithium iron manganese phosphate solid solution is suitable for a stable electrochemical window of a traditional electrolyte system, and the migration rate and the cycle performance of Li ions can be improved at the same time. However, the rate capability, cycle performance, etc. of the existing lithium iron manganese phosphate composite material still need to be further improved.

Disclosure of Invention

The application aims to provide an iron-site-doped lithium manganese iron phosphate composite material, a preparation method thereof and a secondary battery, and aims to solve the technical problems that the rate capability, the cycle performance and the like of the lithium manganese iron phosphate composite material are still to be further improved to a certain extent.

In order to achieve the purpose of the application, the technical scheme adopted by the application is as follows:

in a first aspect, the application provides an iron site-doped lithium iron manganese phosphate composite material, the iron site-doped lithium iron manganese phosphate composite material includes a core and a carbon shell layer coated on the outer surface of the core, the core includes a chemical general formula of LiMn _0.6 Fe _0.4-x-y-z-w Mg _x V _y Al _z A _w PO ₄ Wherein A comprises at least one of Ca, ba, ti and Zn; the value ranges of x, y and z are respectively and independently 0-0.05, and the value ranges of x, y and z are not 0,0 and not more than 0.05,0 and not more than x + y + z + w and not more than 0.15 at the same time.

In a second aspect, the application provides a preparation method of an iron-site-doped lithium manganese iron phosphate composite material, comprising the following steps:

according to the chemical formula LiMn _0.6 Fe _0.4-x-y-z-w Mg _x V _y Al _z A _w PO ₄ The method comprises the following steps of (1) obtaining raw material components comprising a lithium source, a manganese source, a phosphorus source, an iron source, an A source, a magnesium source, a vanadium source and an aluminum source according to the stoichiometric ratio of the elements, dissolving the raw material components in a solvent, adding a first organic carbon source for mixing reaction, drying and crushing to obtain precursor powder; wherein the A source comprises at least one of a Ca source, a Ba source, a Ti source and a Zn source; the value ranges of x, y and z are respectively and independently 0-0.05, and the value ranges of x, y and z are not 0,0, w is more than or equal to 0.05,0, x + y + z + w is more than or equal to 0.15 at the same time;

and in an inert atmosphere, performing first sintering treatment on the precursor powder, mixing the precursor powder with a second organic carbon source for granulation, and performing second sintering treatment to obtain the iron-site-doped lithium manganese iron phosphate composite material.

In a third aspect, the present application provides a secondary battery comprising a positive electrode, a negative electrode, a separator, an electrolyte; the positive electrode comprises the iron-site-doped lithium manganese iron phosphate composite material or the iron-site-doped lithium manganese iron phosphate composite material prepared by the method.

The iron site-doped lithium manganese iron phosphate composite material provided by the first aspect of the application is of a core-shell structure, and the iron site of the core active material is doped with at least one metal element of Mg, V and Al, wherein the Mg ²⁺ Radius between Fe ²⁺ 、Fe ³⁺ The crystal structure is stabilized in the charge-discharge process between the radii; vanadium and aluminum have valence states higher than that of iron and the ionic radius is similar to that of iron, and high valence doping enables Fe ²⁺ /Fe ³⁺ Coexisting to improve the intrinsic conductivity of the material. Meanwhile, the iron site is also doped with at least one A element of Ca, ba, ti and Zn, wherein the titanium doping can reduce the resistance of electron transmission at an electrode/solution interface and stabilize crystal lattice, and is beneficial to the cycle and rate performance of the material; the doping of calcium and barium is beneficial to controlling the shape and the particle size of the material, and the diffusion rate of lithium ions can be improved; the zinc doping can improve the conductivity of the material and the electrical property of the lithium iron manganese phosphate composite material. After bulk phase doping, the crystal lattice shrinks, so that the unit cell parameter of the doped lithium manganese iron phosphate is reduced, the deformation amount of the crystal lattice is reduced in the charging and discharging process, the crystal lattice structure is stabilized, and the polarization degree of the material is reduced. And after the elements are doped, the degree of arrangement of the active material is increased, so that more lattice defects are formed, and more excess electrons can be generated by high-valence metal ions, so that the conductivity of the material is improved. In addition, the carbon shell layer coated on the outer surface of the core is beneficial to improving the conductivity of the composite material, and can limit the particle size of the composite material, so that the particle size of the composite material is small, the uniformity is high, the active specific surface area is large, the compaction density of the lithium iron manganese phosphate composite material is improved, and the specific capacity of the composite material is improved.

According to the preparation method of the iron-site-doped lithium manganese iron phosphate composite material, after raw material components are obtained according to the stoichiometric ratio of an active material, the raw material components are dissolved in a solvent together, then an organic carbon source is added for mixing reaction, in the process, the organic carbon source and the slurry of the raw material components are subjected to oxidation-reduction reaction, a large amount of heat is released, most of water in a mixing system is removed through evaporation, and a multi-component-doped lithium manganese iron phosphate precursor is formed. Further drying to remove the solvent, reducing the energy consumption of the subsequent sintering treatment, and crushing into precursor powder. And then, carrying out first sintering treatment in an inert atmosphere, wherein an organic carbon source doped in the precursor plays a role in consuming acid radical ions in the precursor and protecting the lithium manganese iron phosphate from being oxidized during the sintering process, and the iron-doped lithium manganese iron phosphate is gradually formed. And then, mixing and granulating the product of the first sintering treatment and a second organic carbon source to ensure that the second organic carbon source is uniformly attached to the surface of the first sintering product, then carrying out second sintering treatment to thoroughly form the iron-site-doped lithium manganese iron phosphate active material, carbonizing the organic carbon source into a carbon material, and forming a uniform carbon coating layer on the surface of the iron-site-doped lithium manganese iron phosphate active particles to obtain the iron-site-doped lithium manganese iron phosphate composite material. According to the prepared iron-doped lithium manganese iron phosphate composite material, the iron site in the lithium manganese iron phosphate material is doped with at least one metal element of Mg, V and Al and at least one A element of Ca, ba, ti and Zn, so that the lithium manganese iron phosphate active material with iron sites co-doped by multiple metal ions has better rate capability and cycling stability. In addition, the carbon shell layer not only improves the conductivity of the composite material, but also improves the compacted density and specific capacity of the iron-doped lithium manganese iron phosphate composite material.

According to the secondary battery provided by the third aspect of the application, as the positive electrode contains the iron-site-doped lithium manganese iron phosphate composite material, the composite material has comprehensive properties of excellent rate performance, high cycle stability, high specific capacity and the like. Thereby being beneficial to improving the electrochemical properties of the secondary battery, such as energy density, cycling stability, multiplying power and the like.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments or the prior art descriptions will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present application, and it is obvious for those skilled in the art to obtain other drawings without creative efforts.

Fig. 1 is a schematic flow chart of a preparation method of an iron-site-doped lithium iron manganese phosphate composite material provided in an embodiment of the present application.

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects to be solved by the present application more clearly apparent, the present application is further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present application and are not intended to limit the present application.

In this application, the term "and/or" describes an association relationship of associated objects, meaning that there may be three relationships, e.g., a and/or B, which may mean: a is present alone, A and B are present simultaneously, and B is present alone. Wherein A and B can be singular or plural. The character "/" generally indicates that the former and latter associated objects are in an "or" relationship.

In the present application, "at least one" means one or more, "a plurality" means two or more. "at least one of the following" or similar expressions refer to any combination of these items, including any combination of the singular or plural items. For example, "at least one (one) of a, b, or c," or "at least one (one) of a, b, and c," may each represent: a, b, c, a-b (i.e., a and b), a-c, b-c, or a-b-c, wherein a, b, and c may be single or plural, respectively.

It should be understood that, in various embodiments of the present application, the sequence numbers of the above-mentioned processes do not mean the execution sequence, some or all of the steps may be executed in parallel or executed sequentially, and the execution sequence of each process should be determined by its function and inherent logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

The terminology used in the embodiments of the present application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used in the examples of this application and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

The weight of the related components mentioned in the embodiments of the present specification may not only refer to the specific content of each component, but also represent the proportional relationship of the weight of each component, and therefore, the proportional enlargement or reduction of the content of the related components according to the embodiments of the present specification is within the scope disclosed in the embodiments of the present specification. Specifically, the mass in the examples of the present application may be in units of mass known in the chemical industry, such as μ g, mg, g, and kg.

The terms "first" and "second" are used for descriptive purposes only and are used for distinguishing purposes such as substances from one another, and are not to be construed as indicating or implying relative importance or implying any number of technical features indicated. For example, a first XX may also be referred to as a second XX, and similarly, a second XX may also be referred to as a first XX, without departing from the scope of embodiments of the present application. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature.

A first aspect of the embodiments of the present application provides an iron site-doped lithium iron manganese phosphate composite material, which includes a core and a carbon shell layer coated on an outer surface of the core, where the core includes a chemical general formula of LiMn _0.6 Fe _0.4-x-y-z-w Mg _x V _y Al _z A _w PO ₄ Wherein A comprises at least one of Ca, ba, ti and Zn; the value ranges of x, y and z are respectively and independently 0-0.05, and the value ranges of x, y and z are not 0,0 and not more than 0.05,0 and not more than x + y + z + w and not more than 0.15 at the same time. .

The iron site-doped lithium iron manganese phosphate composite material provided by the first aspect of the embodiment of the application is of a core-shell structure, and the chemical general formula of the core is LiMn _0.6 Fe _0.4-x-y-z-w Mg _x V _y Al _z A _w PO ₄ The value ranges of x, y and z are respectively and independently 0-0.05, and x, y and z are not 0 at the same time, and the iron site is doped with at least one metal element of Mg, V and Al, wherein Mg ²⁺ Radius between Fe ²⁺ 、Fe ³⁺ The crystal structure is stabilized in the charge-discharge process among the radiuses; high valence of vanadium and aluminumIn the iron with the ion radius close to that of the iron, the high-valence doping ensures that Fe ²⁺ /Fe ³⁺ Coexisting, improving the intrinsic conductivity of the material. Meanwhile, the iron site is also doped with at least one A element of Ca, ba, ti and Zn, wherein the titanium doping can reduce the resistance of electron transmission at an electrode/solution interface and stabilize crystal lattice, and is beneficial to the cycle and rate performance of the material; the doping of calcium and barium is beneficial to controlling the shape and the particle size of the material, and the diffusion rate of lithium ions can be improved; the zinc doping can improve the conductivity of the material and the electrical property of the lithium iron manganese phosphate composite material. In the active material of the iron-site-doped lithium manganese iron phosphate composite material in the embodiment of the application, at least one metal element of Mg, V and Al and at least one A element of Ca, ba, ti and Zn are simultaneously doped in the iron site, the cell parameters are all lower than those of the iron element, and the crystal lattice shrinks after bulk phase doping, so that the cell parameters of the doped lithium manganese iron phosphate are reduced, the crystal lattice deformation amount is reduced in the charging and discharging process, the crystal lattice structure is stabilized, and the polarization degree of the material is reduced. And after the elements are doped, the degree of arrangement of the active material is increased, so that more lattice defects are formed, and more excess electrons can be generated by high-valence metal ions, so that the conductivity of the material is improved. In addition, the composite material also comprises a carbon shell layer coated on the outer surface of the core, which is not only favorable for improving the conductivity of the composite material, but also can limit the particle size of the composite material, so that the particle size of the composite material is small, the uniformity is high, the active specific surface area is large, and the compaction density of the lithium iron manganese phosphate composite material is favorably improved, thereby improving the specific capacity of the composite material. Therefore, the iron-site-doped lithium manganese iron phosphate composite material has the comprehensive properties of excellent rate capability, high cycle stability, high specific capacity and the like through the synergistic effect of the iron-site-doped lithium manganese iron phosphate active material and the carbon shell layer.

In the active core of the iron site doped lithium manganese iron phosphate composite material in the embodiment of the application, the doping molar ratio of Mg, V and Al is 0-0.05, and the doping molar ratio of Mg, V and Al is not 0,0, W is more than 0 and less than or equal to 0.05, and x + y + z + w is more than 0 and less than or equal to 0.15. Under the doping condition, the rate capability and the cycle stability of the iron-site-doped lithium manganese iron phosphate composite material are improved most beneficially. If the doping content is too high, the proportion of the main element of the lithium iron manganese phosphate is affected, and the specific capacity and the structural stability of the active material are also affected.

In some possible implementations, the chemical formula is LiMn _0.6 Fe _0.4-x-y-z-w Mg _x V _y Al _z A _w PO ₄ In the active material of (2), x, y and z are all larger than 0, namely, the iron site is simultaneously doped with Mg, V and Al and at least one of Ca, ba, ti and Zn. Under the doping condition, the unit cell parameters of the doped metal ions are respectively Mg (2 +) 65pm, V (5 +) 54pm, al (3 +) 50pm, ti (4 +) 68pm and Zn (2 +) 74pm, and are all smaller than the substituted Fe (2 +) 76pm, the crystal lattice shrinks after bulk phase doping, the unit cell parameters are reduced, the crystal lattice deformation amount is reduced in the charge-discharge process, the crystal lattice structure is stabilized, and the polarization degree of the material is reduced. The degree of doping can be increased, so that more lattice defects are formed, and the high-valence metal ions can generate more excess electrons, so that the conductivity of the active material is improved. Therefore, the lithium iron manganese phosphate active material with iron sites co-doped with various metal ions has better rate performance and cycle stability.

In some possible implementation manners, the mass percentage of the carbon shell layer is 1% -1.6% based on 100% of the total mass of the iron-site-doped lithium iron manganese phosphate composite material. Under the condition, the mass percentage content of the carbon shell layer is favorable for improving the conductivity of the lithium manganese iron phosphate composite material, and can completely coat the active core material to inhibit the size of the core particles, so that the lithium manganese iron phosphate composite material has small particle size, high uniformity and large active specific surface area, and the compaction density of the composite material is favorably improved. In some embodiments, the total mass of the iron-site-doped lithium iron manganese phosphate composite material is 100%, and the mass percentage of the carbon shell layer may be 1% to 1.2%, 1.2% to 14%, 1.4% to 1.6%, and the like.

In some possible implementations, the thickness of the carbon shell layer is 2nm to 5nm; the carbon shell layer with the thickness can form a complete coating layer on the outer surface of the active core particle, so that the size of the lithium iron manganese phosphate active material particle is effectively inhibited, and the particle size is small and the uniformity is high. Moreover, the carbon shell layer with the thickness is also beneficial to improving the conductivity of the lithium iron manganese phosphate composite material. In some embodiments, the thickness of the carbon shell layer may be 2nm to 3nm, 3nm to 4nm, 4nm to 5nm, and the like.

In some possible implementation modes, the primary particle size of the iron-site-doped lithium manganese iron phosphate composite material is 50 nm-300 nm; the primary particle size refers to the size of a single particle of the iron-doped lithium iron manganese phosphate composite material under a microscopic test condition, namely the particle size of the single microscopic particle. The iron-site-doped lithium manganese iron phosphate composite material provided by the embodiment of the application has the advantages that the primary particle size is nanoscale, the particle size is small, the active specific surface area is large, and the application of the composite material to a positive electrode material of a secondary battery is beneficial to improving the ion embedding and separating efficiency, so that the multiplying power performance and the cycle performance of the secondary battery are improved. In some embodiments, the iron site-doped lithium iron manganese phosphate composite may have a primary particle size of 50nm to 100nm, 100nm to 150nm, 150nm to 200nm, 200nm to 250nm, 250nm to 300nm, and the like.

In some possible implementation manners, the primary particle size of the iron-site-doped lithium manganese iron phosphate composite material is 0.5-1.5 μm; the secondary particle size refers to the size of a group-packed particle formed by stacking single particles of the iron-site-doped lithium iron manganese phosphate composite material, namely the particle size of a macro particle formed by stacking a plurality of primary particles. The iron-site-doped lithium manganese iron phosphate composite material provided by the embodiment of the application has the advantages of small macro-particle size and high uniformity, and is favorable for improving the compaction density of the iron-site-doped lithium manganese iron phosphate composite material, and the application of the composite material to a positive electrode material of a secondary battery is favorable for improving the energy density. In some embodiments, the primary particle size of the iron site doped lithium iron manganese phosphate composite material may be 0.5 μm to 0.8 μm, 0.8 μm to 1.0 μm, 1.0 μm to 1.2 μm, 1.2 μm to 1.5 μm, and the like.

The iron-site-doped lithium manganese iron phosphate composite material provided by the embodiment of the application can be prepared by the following embodiment method.

As shown in fig. 1, a second aspect of the embodiments of the present application provides a method for preparing an iron-site-doped lithium iron manganese phosphate composite material, including the following steps:

s10, according to a chemical general formula LiMn _0.6 Fe _0.4-x-y-z-w Mg _x V _y Al _z A _w PO ₄ The stoichiometric ratio of each element in the raw material mixture is obtained, raw material components comprising a lithium source, a manganese source, a phosphorus source, an iron source, an A source, a magnesium source, a vanadium source and an aluminum source are obtained, the raw material components are dissolved in a solvent, a first organic carbon source is added for mixing reaction, and the mixture is dried and crushed to obtain precursor powder; wherein, the A source comprises at least one of a Ca source, a Ba source, a Ti source and a Zn source; the value ranges of x, y and z are respectively and independently 0-0.05, and the value ranges of x, y and z are not 0,0, w is more than or equal to 0.05,0, x + y + z + w is more than or equal to 0.15 at the same time;

s20, in an inert atmosphere, performing first sintering treatment on the precursor powder, mixing the precursor powder with a second organic carbon source for granulation, and performing second sintering treatment to obtain the iron-site-doped lithium manganese iron phosphate composite material.

In the second aspect of the embodiments of the present application, a method for preparing an iron-doped lithium iron manganese phosphate composite material is provided according to a chemical general formula LiMn _0.6 Fe _0.4-x-y-z-w Mg _x V _y Al _z A _w PO ₄ After raw material components are obtained according to the stoichiometric ratio of the elements, the raw material components are dissolved in a solvent together, then an organic carbon source is added for mixing reaction, in the process, the organic carbon source and the slurry of the raw material components are subjected to redox reaction, a large amount of heat is released, most of water in a mixing system is removed through evaporation, and a multi-component doped lithium iron manganese phosphate precursor is formed. Further drying to remove the solvent, reducing the energy consumption of the subsequent sintering treatment, and crushing into precursor powder. And then carrying out first sintering treatment in an inert atmosphere, wherein the organic carbon source doped in the precursor simultaneously plays a role in consuming acid radical ions in the precursor and protecting the lithium manganese iron phosphate from oxidation in the sintering process, and the iron-doped lithium manganese iron phosphate is gradually formed. Then, mixing and granulating the product of the first sintering treatment and a second organic carbon source to ensure that the second organic carbon source is uniformly attached to the surface of the first sintering product, and then performing a second sintering treatment to thoroughly form an iron-site-doped lithium manganese iron phosphate active material, wherein the organic carbon source is used for preparing the organic carbon sourceAnd carbonizing the mixture into a carbon material, and forming a uniform carbon coating layer on the surface of the iron-doped lithium manganese iron phosphate active particles to obtain the iron-doped lithium manganese iron phosphate composite material. The preparation process is simple, the preparation method is suitable for industrial large-scale production and application, the prepared iron-site-doped lithium manganese iron phosphate composite material is prepared by doping at least one metal element of Mg, V and Al and at least one element A of Ca, ba, ti and Zn into an iron site in a lithium manganese iron phosphate material, and the lithium manganese iron phosphate active material after iron site co-doping of multiple metal ions has better rate capability and cycling stability. In addition, the carbon shell layer not only improves the conductivity of the composite material, but also ensures that the particle size of the composite material is small, the uniformity is high, the active specific surface area is large, and the compaction density and the specific capacity of the iron-doped lithium manganese iron phosphate composite material are improved.

In the above step S10, liMn is represented by the general chemical formula _0.6 Fe _0.4-x-y-z-w Mg _x V _y Al _z A _w PO ₄ Wherein A comprises at least one of Ca, ba, ti and Zn; the value ranges of x, y and z are respectively and independently 0-0.05, and the value ranges of x, y and z are not 0,0 and not more than 0.05,0 and not more than x + y + z + w and not more than 0.15 at the same time.

In some possible implementations, the chemical formula is LiMn _0.6 Fe _0.4-x-y-z-w Mg _x V _y Al _z A _w PO ₄ In the active material of (1), x, y and z are all greater than 0. Under the doping condition, the specific capacity, the rate capability and the cycling stability of the lithium iron manganese phosphate material can be better improved.

In some possible implementations, the solvent includes water and/or nitric acid.

In some possible implementations, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium acetate, lithium oxalate, lithium chloride.

In some possible implementations, the manganese source includes at least one of manganomanganic oxide, manganese dioxide, manganese carbonate, manganese oxalate, manganese lactate, manganese chloride, manganese sulfate, manganese nitrate, manganese dioxide.

In some possible implementations, the source of phosphorus includes at least one of phosphoric acid, diammonium phosphate, and monoammonium phosphate.

In some possible implementations, the iron source includes at least one of ferrous oxalate, ferric oxalate, ferrous acetate, ferric acetate, ferrous nitrate, ferric phosphate, ferric oxide, ferrous sulfate, ferric sulfate, ferrous chloride, ferric chloride.

In some possible implementations, the magnesium source includes at least one of magnesium nitrate, magnesium chloride, magnesium carbonate, magnesium sulfate.

In some possible implementations, the source of vanadium includes NH ₄ VO ₃ At least one of vanadium nitrate and vanadium pentoxide.

In some possible implementations, the aluminum source includes at least one of aluminum nitrate, aluminum chloride, aluminum carbonate, aluminum sulfate.

In some possible implementations, the Ca source includes at least one of calcium hydroxide, calcium oxide, calcium salt.

In some possible implementations, the Ba source includes at least one of barium hydroxide, barium oxide, barium salt.

In some possible implementations, the Ti source includes at least one of titanium hydroxide, titanium oxide, titanium salt.

In some possible implementations, the Zn source includes at least one of zinc powder, zinc chloride, zinc sulfate, zinc gluconate.

The raw material components of the lithium source, the manganese source, the phosphorus source, the iron source, the magnesium source, the vanadium source, the aluminum source, the Ca source, the Ba source, the Ti source, the Zn source and the like obtained in the embodiment of the application have good solubility, can be uniformly and stably dissolved in the solvent, are favorable for preparing the multi-component doped lithium manganese iron phosphate precursor, and then are sintered to prepare the iron-site doped lithium manganese iron phosphate.

In some possible implementations, the first organic carbon source added after dissolving the raw material components in the solvent includes at least one of citric acid, glucose, sucrose, soluble starch, salicylic acid, tartaric acid, oxalic acid, polysorbate, polyethylene glycol, sorbitan fatty acid, stearic acid, glycerol fatty acid ester, amino acid. On one hand, the organic carbon sources have better solubility in the solvent, can be uniformly and stably dispersed in the solvent, can generate oxidation-reduction reaction with acid ions in the solution in the synthesis process of the lithium iron manganese phosphate precursor, emit a large amount of heat, and evaporate to remove the solvent in the reaction system to form lithium iron manganese phosphate precursor particles. On the other hand, the first sintering process of the lithium manganese iron phosphate precursor simultaneously plays a role in consuming acid ions in the precursor and protecting the lithium manganese iron phosphate from being oxidized.

In some possible implementations, the mass ratio of the feedstock components to the first organic carbon source is 1: (0.1-0.2); the organic carbon source with the mass ratio fully ensures the synthesis of the multi-component doped lithium iron manganese phosphate precursor, and plays the roles of a sufficient reducing agent and an oxidation protective agent in the first sintering process. If the carbon source is added too much, the particles after the first sintering are agglomerated to form particles with larger particle size, and the subsequent treatment process is not beneficial to opening the agglomerates and preparing the composite particles with small particle size. In some embodiments, the mass ratio of the feedstock components to the first organic carbon source may be 1: (0.1-0.12), 1: (0.12 to 0.15), 1: (0.15 to 0.17), 1: (0.17-0.2), and the like. In some embodiments, the mass ratio of the feedstock components to the first organic carbon source is 1.

In some possible implementations, in the step S20, the conditions of the first sintering process include: sintering the mixture for 5 to 7 hours in an inert atmosphere at the temperature of between 500 and 600 ℃ to ensure that the precursor powder is gradually converted into iron-doped lithium manganese iron phosphate. If the temperature is too high, the primary particles of lithium iron manganese phosphate will be too large, which is not favorable for preparing particles with small particle size and high uniformity. In some embodiments, the first sintering temperature condition may be 500 to 550 ℃, 550 to 600 ℃, etc., and the sintering time period may be 5 to 6 hours, 6 to 7 hours, etc.

In some possible implementations, the step of mixing granulation includes: and preparing a mixed slurry from the product of the first sintering treatment and a second organic carbon source, uniformly coating the organic carbon source on the surface of the product of the first sintering treatment, effectively limiting the growth of lithium iron manganese phosphate particles in the subsequent sintering process, grinding and sieving, refining the particle size, improving the uniformity of the particle size, and spray-drying to ensure that the organic carbon source is tightly attached to the surface of the product of the first sintering treatment to obtain the granulated powder. In some embodiments, after grinding 10 to 20 times, the resulting mixture is passed through a 100-300 mesh screen.

In some possible implementations, the conditions of the second sintering process include: roasting for 8-10 hours in an inert atmosphere at the temperature of 650-750 ℃, so that the multi-component doped lithium iron manganese phosphate precursor powder is fully and thoroughly converted into iron-site doped lithium iron manganese phosphate particles, and simultaneously, an organic carbon source attached to the surface is carbonized into a carbon material, and a compact carbon coating layer is formed in situ on the surface of the lithium iron manganese phosphate particles. If the sintering temperature is too high, the particle size of the composite particles is increased, the specific capacity is reduced, and the pH value is increased, so that the performance of the battery is influenced; if the sintering temperature is too low; the particle size of the composite particles is smaller, the compaction is lower, the battery manufacturing is influenced, and the battery manufacturing pole piece is easy to crack. In some embodiments, the second sintering temperature may be 650 to 700 ℃, 700 to 750 ℃, and the like, and the sintering time may be 8 to 9 hours, 9 to 10 hours, and the like.

In some possible implementations, the second organic carbon source includes at least one of citric acid, glucose, sucrose, soluble starch, salicylic acid, tartaric acid, oxalic acid, polysorbate, polyethylene glycol, sorbitan fatty acid, stearic acid, glycerol fatty acid ester, amino acid. The organic carbon sources have good solubility, can be uniformly and stably attached to the surface of a primary sintering product, and effectively inhibit the growth of lithium manganese iron phosphate particles in the secondary sintering treatment process, so that the iron site-doped lithium manganese iron phosphate composite material has small particle size, high uniformity, large active specific surface area and excellent conductivity.

In some possible implementations, the mass ratio of the product of the first sintering treatment to the second organic carbon source is 1: (0.03-0.08), the organic carbon source with the proportion can form a complete and compact carbon coating layer on the surface of the iron-doped lithium manganese iron phosphate active core, so that the conductivity of the composite material is improved, and the composite material has small particle size and large specific surface area of activity. In some embodiments, the mass ratio of the product of the first sintering process to the second organic carbon source may be 1: (0.03 to 0.05), 1: (0.04 to 0.06), 1: (0.05-0.08), and the like. In some embodiments, the mass ratio of the product of the first sintering process to the second organic carbon source is 1.

A third aspect of the embodiments of the present application provides a secondary battery including a positive electrode, a negative electrode, a separator, an electrolyte solution; the anode comprises the iron-site-doped lithium manganese iron phosphate composite material or the iron-site-doped lithium manganese iron phosphate composite material prepared by the method.

In the secondary battery provided by the third aspect of the embodiment of the application, since the positive electrode contains the iron-site-doped lithium manganese iron phosphate composite material, the composite material has excellent comprehensive properties such as rate performance, high cycling stability performance, high specific capacity and the like. Thereby being beneficial to improving the electrochemical properties of the secondary battery, such as energy density, cycling stability, multiplying power and the like.

In some possible implementations, the negative electrode of the secondary battery includes, but is not limited to, graphite, soft carbon (e.g., coke, etc.), hard carbon, or other carbon materials, or nitrides, tin-based oxides, tin alloys, and nano-negative electrode materials, etc.

In some possible implementations, the separator includes at least one material of polypropylene fibers, polyacrylonitrile fibers, polyvinyl formal fibers, poly (ethylene glycol terephthalate), polyethylene terephthalate, polyamide fibers, and poly (paraphenylene terephthalamide).

In some possible implementations, the electrolyte includes Na-containing ⁺ 、K ⁺ 、NH ⁴⁺ An aqueous solution of a soluble salt of at least one of (a) and (b).

In order to make the details and operations of the foregoing embodiments of the present application clearly understood by those skilled in the art, and to make the progress of the iron-doped lithium iron manganese phosphate composite material and the preparation method thereof apparent in the examples of the present application, the foregoing technical solutions are illustrated in the following by using a plurality of examples.

Example 1

A lithium iron manganese phosphate composite material with Mg, V and Ti doped at iron sites is prepared by the following steps:

1. preparing a precursor: 738.9g Li ₂ CO ₃ 1320.56g NH ₄ H ₂ PO ₄ 967.44g of Fe (NO) ₃ ) ₃ 1073.688g Mn (NO) ₃ ) ₂ 89.474g of Mg (NO) ₃ ) ₂ 180.4707g V (NO) ₃ ) ₅ 39.93g of TiO ₂ After being dissolved and mixed uniformly according to the proportion, 882.09314g of glucose solution is added to obtain mixed solution, and the mixed solution is dried and crushed after being mixed uniformly and ball-milled to obtain a precursor.

2. And (3) sintering: and (2) pretreating the prepared precursor at 550 ℃ for 6 hours at constant temperature in a nitrogen atmosphere, supplementing 50g of glucose solution again, stirring to prepare mixed slurry, sanding for 20 times, sieving by using a 300-mesh sieve, spray-drying, roasting the dried powder for 10 hours at 700 ℃ in the nitrogen atmosphere, cooling to room temperature, taking out, crushing, and sieving by using the 300-mesh sieve to obtain the lithium manganese iron phosphate composite material with the iron positions doped with Mg, V and Ti.

Example 2

A lithium iron manganese phosphate composite material with Mg, ba and Ti doped at iron sites is prepared by the following steps:

1. preparing a precursor: 738.9g of Li ₂ CO ₃ 1320.56g NH ₄ H ₂ PO ₄ 967.44g of Fe (NO) ₃ ) ₃ 1073.688g Mn (NO) ₃ ) ₂ 89.474g of Mg (NO) ₃ ) ₂ 130.67g Ba (NO) ₃ ) ₂ 39.93g of TiO ₂ Dissolving and mixing uniformly according to a proportion, adding 872.1327g of glucose solution to obtain a mixed solution, mixing uniformly, ball-milling, drying and crushing to obtain a precursor.

2. And (3) sintering: and (2) pretreating the prepared precursor at 550 ℃ for 6 hours at constant temperature in a nitrogen atmosphere, supplementing 50g of glucose solution again, stirring to prepare mixed slurry, sanding for 20 times, sieving by using a 300-mesh sieve, spray-drying, roasting the dried powder for 10 hours at 700 ℃ in the nitrogen atmosphere, cooling to room temperature, taking out, crushing, and sieving by using the 300-mesh sieve to obtain the lithium manganese iron phosphate composite material with Mg, ba and Ti doped at iron sites.

Example 3

A lithium iron manganese phosphate composite material with iron doped V, ba and Ti is prepared by the following steps:

1. preparing a precursor: 738.9g of Li ₂ CO ₃ 1320.56g NH ₄ H ₂ PO ₄ 967.44g Fe (NO) ₃ ) ₃ 1073.688g Mn (NO) ₃ ) ₂ 180.4707g of V (NO) ₃ ) ₅ 130.67g of Ba (NO) ₃ ) ₂ 39.93g of TiO ₂ After being dissolved and mixed uniformly according to the proportion, 890.33204g of glucose solution is added to obtain mixed solution, and the mixed solution is dried and crushed after being mixed uniformly and ball-milled to obtain a precursor.

2. And (3) sintering: and (2) after the prepared precursor is pretreated at the constant temperature of 550 ℃ for 6 hours in a nitrogen atmosphere, supplementing 50g of glucose solution again, stirring to prepare mixed slurry, sanding for 20 times, sieving by using a 300-mesh sieve, spray-drying, roasting the dried powder for 10 hours in a nitrogen atmosphere at the temperature of 700 ℃, cooling to room temperature, taking out, crushing, and sieving by using a 300-mesh sieve to obtain the iron site doped V, ba and Ti lithium iron manganese phosphate composite material.

Example 4

A manganese iron phosphate lithium composite material codoped with iron site V, mg, ba and Ti is prepared by the following steps:

1. preparing a precursor: 738.9g of Li ₂ CO ₃ 1320.56g NH ₄ H ₂ PO ₄ 967.44g Fe (NO) ₃ ) ₃ 1073.688g Mn (NO) ₃ ) ₂ 89.474g Mg (NO) ₃ ) ₂ 180.4707g of V (NO) ₃ ) ₅ 130.67g Ba (NO) ₃ ) ₂ 39.93g of TiO ₂ After being dissolved and mixed uniformly according to the proportion, 908.22684g of glucose solution is added to obtain mixed solution, and the mixed solution is dried and crushed after being mixed uniformly and ball-milled to obtain a precursor.

2. And (3) sintering: and (2) pretreating the prepared precursor at 550 ℃ for 6 hours at constant temperature in a nitrogen atmosphere, supplementing 50g of glucose solution again, stirring to prepare mixed slurry, sanding for 20 times, sieving with a 300-mesh sieve, spray-drying, roasting the dried powder at 700 ℃ for 10 hours in the nitrogen atmosphere, cooling to room temperature, taking out, crushing, and sieving with the 300-mesh sieve to obtain the iron site V, mg, ba and Ti co-doped lithium manganese iron phosphate composite material.

Example 5

A manganese lithium iron phosphate composite material codoped with iron site Mg, V, al and Ti is prepared by the following steps:

1. preparing a precursor: 738.9g of Li ₂ CO ₃ 1320.56g NH ₄ H ₂ PO ₄ 967.44g of Fe (NO) ₃ ) ₃ 1073.688g Mn (NO) ₃ ) ₂ 89.474g of Mg (NO) ₃ ) ₂ 180.4707g of V (NO) ₃ ) ₅ 106.498g of Al (NO) ₃ ) ₃ 39.93g of TiO ₂ Dissolving and mixing uniformly according to a proportion, adding 906.4g of glucose solution to obtain a mixed solution, mixing uniformly, carrying out ball milling, drying and crushing to obtain a precursor.

2. And (3) sintering: and (2) pretreating the prepared precursor at 550 ℃ for 6 hours at constant temperature in a nitrogen atmosphere, supplementing 50g of glucose solution again, stirring to prepare mixed slurry, sanding for 20 times, sieving with a 300-mesh sieve, spray-drying, roasting the dried powder at 700 ℃ for 10 hours in the nitrogen atmosphere, cooling to room temperature, taking out, crushing, and sieving with the 300-mesh sieve to obtain the iron-position Mg, V, al and Ti co-doped lithium manganese iron phosphate composite material.

Example 6

A manganese lithium iron phosphate composite material codoped with iron position Mg, V, al, ti, ba and Zn is prepared by the following steps:

1. preparing a precursor: 738.9g of Li ₂ CO ₃ 1320.56g NH ₄ H ₂ PO ₄ 967.44g of Fe (NO) ₃ ) ₃ 1073.688g Mn (NO) ₃ ) ₂ 89.474g of Mg (NO) ₃ ) ₂ 180.4707g V (NO) ₃ ) ₅ 130.67g of Al (NO) ₃ ) ₃ 39.93g of TiO ₂ 130.67g of Ba (NO) ₃ ) ₂ 94.7g of Zn (NO) ₃ ) ₂ After being dissolved and mixed evenly according to the proportionAnd adding 927.16684g of glucose solution to obtain a mixed solution, uniformly mixing, performing ball milling, drying and crushing to obtain a precursor.

2. And (3) sintering: and (2) pretreating the prepared precursor at 550 ℃ for 6 hours at constant temperature in a nitrogen atmosphere, supplementing 50g of glucose solution again, stirring to prepare mixed slurry, sanding for 20 times, sieving with a 300-mesh sieve, spray-drying, roasting the dried powder at 700 ℃ for 10 hours in the nitrogen atmosphere, cooling to room temperature, taking out, crushing, and sieving with the 300-mesh sieve to obtain the iron-position Mg, V, al, ti, ba and Zn co-doped lithium manganese iron phosphate composite material.

Comparative example 1

An undoped lithium manganese iron phosphate composite material is prepared by the following steps:

1. preparing a precursor: 738.9g Li ₂ CO ₃ 1320.56g NH ₄ H ₂ PO ₄ 967.44g of Fe (NO) ₃ ) ₃ 1073.688g Mn (NO) ₃ ) ₂ Dissolving and mixing uniformly according to a proportion, adding 820.1176g of glucose solution to obtain a mixed solution, mixing uniformly, ball-milling, drying and crushing to obtain a precursor.

2. And (3) sintering: and (2) pretreating the prepared precursor at 550 ℃ for 6 hours at constant temperature in a nitrogen atmosphere, supplementing 50g of glucose solution again, stirring to prepare mixed slurry, sanding for 20 times, sieving with a 300-mesh sieve, spray-drying, roasting the dried powder for 10 hours at 700 ℃ in the nitrogen atmosphere, cooling to room temperature, taking out, crushing, and sieving with the 300-mesh sieve to obtain the undoped lithium manganese iron phosphate composite material.

Comparative example 2

A lithium iron manganese phosphate composite material with an iron site only doped with Mg is prepared by the following steps:

1. preparing a precursor: 738.9g of Li ₂ CO ₃ 1320.56g NH ₄ H ₂ PO ₄ 967.44g of Fe (NO) ₃ ) ₃ 1073.688g Mn (NO) ₃ ) ₂ 89.474g of Mg (NO) ₃ ) ₂ Dissolving and mixing uniformly according to a proportion, adding 838.0124g of glucose solution to obtain a mixed solution, mixing uniformly, ball-milling, drying and crushing to obtain a precursorAnd (3) a body.

2. And (3) sintering: and (2) pretreating the prepared precursor at 550 ℃ for 6 hours at constant temperature in a nitrogen atmosphere, supplementing 50g of glucose solution again, stirring to prepare mixed slurry, sanding for 20 times, sieving with a 300-mesh sieve, spray-drying, roasting the dried powder for 10 hours at 700 ℃ in the nitrogen atmosphere, cooling to room temperature, taking out, crushing, and sieving with the 300-mesh sieve to obtain the lithium manganese iron phosphate composite material with the iron position only doped with Mg.

Comparative example 3

A lithium iron manganese phosphate composite material with only vanadium doped at an iron site is prepared by the following steps:

1. preparing a precursor: 738.9g of Li ₂ CO ₃ 1320.56g NH ₄ H ₂ PO ₄ 967.44g of Fe (NO) ₃ ) ₃ 1073.688g Mn (NO) ₃ ) ₂ 180.4707g of V (NO) ₃ ) ₅ Dissolving and mixing uniformly according to a proportion, adding 856.21174g of glucose solution to obtain a mixed solution, mixing uniformly, ball-milling, drying and crushing to obtain a precursor.

2. And (3) sintering: and (2) pretreating the prepared precursor at 550 ℃ for 6 hours at constant temperature in a nitrogen atmosphere, supplementing 50g of glucose solution again, stirring to prepare mixed slurry, sanding for 20 times, sieving with a 300-mesh sieve, spray-drying, roasting the dried powder for 10 hours at 700 ℃ in the nitrogen atmosphere, cooling to room temperature, taking out, crushing, and sieving with the 300-mesh sieve to obtain the lithium manganese iron phosphate composite material with only vanadium doped at the iron site.

Comparative example 4

A lithium iron manganese phosphate composite material with an iron site only doped with titanium is prepared by the following steps:

1. preparing a precursor: 738.9g of Li ₂ CO ₃ 1320.56g NH ₄ H ₂ PO ₄ 967.44g of Fe (NO) ₃ ) ₃ 1073.688g Mn (NO) ₃ ) ₂ 39.93g of TiO ₂ The HCl is dissolved and mixed evenly according to the proportion, 828.1042g of glucose solution is added to obtain mixed solution, and the mixed solution is dried and crushed after being mixed evenly and ball milled to obtain the precursor.

2. And (3) sintering: and (2) pretreating the prepared precursor at 550 ℃ for 6 hours at constant temperature in a nitrogen atmosphere, supplementing 50g of glucose solution again, stirring to prepare mixed slurry, sanding for 20 times, sieving with a 300-mesh sieve, spray-drying, roasting the dried powder for 10 hours at 700 ℃ in the nitrogen atmosphere, cooling to room temperature, taking out, crushing, and sieving with the 300-mesh sieve to obtain the lithium manganese iron phosphate composite material with the iron site only doped with titanium.

Comparative example 5

A lithium iron manganese phosphate composite material with an iron site only doped with barium is prepared by the following steps:

1. preparing a precursor: 738.9g Li ₂ CO ₃ 1320.56g NH ₄ H ₂ PO ₄ 967.44g of Fe (NO) ₃ ) ₃ 1073.688g Mn (NO) ₃ ) ₂ 130.67g of Ba (NO) ₃ ) ₂ Dissolving and mixing uniformly according to a proportion, adding 846.2513g of glucose solution to obtain a mixed solution, mixing uniformly, ball-milling, drying and crushing to obtain a precursor.

2. And (3) sintering: and (2) after the prepared precursor is pretreated at the constant temperature of 550 ℃ for 6 hours in a nitrogen atmosphere, supplementing 50g of glucose solution again, stirring to prepare mixed slurry, sanding for 20 times, sieving by using a 300-mesh sieve, spray-drying, roasting the dried powder for 10 hours in the nitrogen atmosphere at the temperature of 700 ℃, cooling to room temperature, taking out, crushing, and sieving by using the 300-mesh sieve to obtain the lithium manganese iron phosphate composite material with the iron position only doped with barium.

Comparative example 6

A lithium iron manganese phosphate composite material with Mg and V doped at iron sites is prepared by the following steps:

1. preparing a precursor: 738.9g Li ₂ CO ₃ 1320.56g NH ₄ H ₂ PO ₄ 967.44g of Fe (NO) ₃ ) ₃ 1073.688g Mn (NO) ₃ ) ₂ 89.474g of Mg (NO) ₃ ) ₂ 180.4707g of V (NO) ₃ ) ₅ Dissolving and mixing uniformly according to a proportion, adding 874.10654g of glucose solution to obtain a mixed solution, mixing uniformly, ball-milling, drying and crushing to obtain a precursor.

2. And (3) sintering: and (2) after the prepared precursor is pretreated at the constant temperature of 550 ℃ for 6 hours in a nitrogen atmosphere, supplementing 50g of glucose solution again, stirring to prepare mixed slurry, sanding for 20 times, sieving by using a 300-mesh sieve, spray-drying, roasting the dried powder for 10 hours in a nitrogen atmosphere at the temperature of 700 ℃, cooling to room temperature, taking out, crushing, and sieving by using a 300-mesh sieve to obtain the lithium manganese iron phosphate composite material with Mg and V doped at the iron site.

Further, in order to verify the advancement of the embodiments of the present application, the following performance tests were respectively performed on the lithium iron manganese phosphate composite materials prepared in the respective embodiments and comparative examples:

1. powder pressure test: the compaction density and the surface resistivity of the lithium iron manganese phosphate composite material finished products of the examples and the comparative examples are respectively measured by weighing 1g of the finished products under the condition that the pressure is 10Mpa (through a 3T pressure sensor), and the test results are shown in the following table 1.

TABLE 1

As can be seen from the test results in table 1, compared to comparative examples 1 to 6, the lithium iron manganese phosphate composite materials prepared in examples 1 to 6 of the present application, in which the iron sites are simultaneously doped with at least one of Mg, V, and Al, and at least one of Ca, ba, ti, and Zn, have the lowest surface resistivity, which may reach 0 Ω · cm. In addition, the test shows that the compacted density is 2.524g/cm ³ Left and right.

2. And (3) testing the battery performance: the lithium iron manganese phosphate composite material provided by the embodiment and the comparative example is adopted to assemble a battery according to the following method:

preparing a positive plate: uniformly mixing the lithium iron manganese phosphate composite materials prepared in comparative examples 1 to 6 and in examples 1 to 6 of the application with SP (conductive carbon black), PVDF (polyvinylidene fluoride) and NMP (N-methyl pyrrolidone) according to a mass ratio of 93.5; and adding the prepared anode slurry on an aluminum foil, uniformly scraping by using a scraper, drying at 130 ℃, and rolling to obtain an anode plate.

The battery assembling process comprises the following steps: the prepared positive electrode is attached to a positive electrode metal shell by using conductive adhesive, a metal lithium sheet is used as a negative electrode, a Celgard 2400 microporous membrane is used as a diaphragm, 1.0mol/L LiPF6 solution is used as electrolyte, and the electrolyte is a mixed solution of Ethylene Carbonate (EC), diethyl carbonate (DEC) and Ethyl Methyl Carbonate (EMC) in a volume ratio of 1.

And testing the electrochemical performances of the button cell, such as the specific energy at 0.1C, the specific energy at 1C, the full charge time of the battery at 0.1C, the low-temperature/normal-temperature cycle performance and the like, by using a LAND electrochemical tester under the conditions that the charge termination voltage is 4.2V and the discharge cut-off voltage is 2.0V. The manganese dissolution after the circulation is tested, and the test results are shown in the following table 2.

TABLE 2

From the test results in table 2, it can be seen that, compared with the manganese lithium iron phosphate composite material not doped in comparative example 1, or the manganese lithium iron phosphate composite material with only one of Mg, V, al, ca, ba, ti, and Zn in iron sites of comparative examples 2 to 5, or the manganese lithium iron phosphate composite material with only magnesium and vanadium in iron sites of comparative example 6, the iron sites co-doped with at least one of Mg, V, and Al prepared in examples 1 to 5 of the present application have better specific energy of 0.1C and 1C, more excellent high and low temperature cycle performance, and lower manganese dissolution rate after the manganese lithium iron phosphate composite material doped with at least one of Ca, ba, ti, and Zn is prepared into a secondary battery. In particular, in example 6, after the secondary battery is made of the manganese iron phosphate composite material co-doped with Mg, V, al, ti, ba and Zn at the iron site, the specific energy reaches 634.2wh/kg at 25 ℃ and 0.1C, the specific energy reaches 560.5wh/kg at 25 ℃ and 1C, and the full charge time of the 0.1C battery only needs 6 hours, which is significantly higher than the test results of each proportion. Moreover, after 5000 cycles of charge and discharge tests in the environment of normal temperature (25 ℃) and low temperature (0 ℃), the button cell can still maintain better capacity, the capacity retention rate reaches 99.5% after 5000 cycles in the environment of normal temperature, and the capacity retention rate reaches 96% after 5000 cycles in the environment of low temperature. In addition, when the dissolution of manganese in the battery positive electrode after the cycle is tested, the dissolution concentration of manganese ions is only 0.01ppm after 5000 cycles of charge and discharge of the button batteries of the embodiments 4 to 6. In addition, examples 1 to 3 also exhibited high specific energy, high rate performance, and excellent cycle stability at room temperature and low temperature, and the manganese ion elution concentration was only about 0.1 ppm. Therefore, the iron site co-doped lithium manganese iron phosphate composite material prepared by the embodiment of the application can simultaneously improve the energy density, the cycling stability and the safety performance of the secondary battery, and prolong the service life of the secondary battery.

The above description is only exemplary of the present application and should not be taken as limiting the present application, as any modification, equivalent replacement, or improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims (10)

1. The iron site-doped lithium manganese iron phosphate composite material is characterized by comprising a core and a carbon shell layer coated on the outer surface of the core, wherein the core comprises a chemical general formula of LiMn _0.6 Fe _0.4-x-y-z-w Mg _x V _y Al _z A _w PO ₄ Wherein A comprises at least one of Ca, ba, ti and Zn; the value ranges of x, y and z are respectively and independently 0-0.05, and the value ranges of x, y and z are not 0,0 and not more than 0.05,0 and not more than x + y + z + w and not more than 0.15 at the same time.

2. The iron-site-doped lithium iron manganese phosphate composite material of claim 1, wherein the chemical formula is LiMn _0.6 Fe _0.4-x-y-z-w Mg _x V _y Al _z A _w PO ₄ Wherein x, y, and z are all greater than 0.

3. The iron-site doped lithium iron manganese phosphate composite material of claim 1 or 2, wherein the carbon shell layer comprises 1-1.6% by mass based on 100% by mass of the total iron-site doped lithium iron manganese phosphate composite material;

and/or the thickness of the carbon shell layer is 2 nm-5 nm;

and/or the primary particle size of the iron site doped lithium iron manganese phosphate composite material is 50 nm-300 nm;

and/or the secondary particle size of the iron site doped lithium iron manganese phosphate composite material is 0.5-1.5 mu m.

4. A preparation method of an iron site-doped lithium iron manganese phosphate composite material is characterized by comprising the following steps:

according to the general formula LiMn _0.6 Fe _0.4-x-y-z-w Mg _x V _y Al _z A _w PO ₄ The method comprises the following steps of (1) obtaining raw material components comprising a lithium source, a manganese source, a phosphorus source, an iron source, an A source, a magnesium source, a vanadium source and an aluminum source according to the stoichiometric ratio of the elements, dissolving the raw material components in a solvent, adding a first organic carbon source for mixing reaction, drying and crushing to obtain precursor powder; wherein the A source comprises at least one of a Ca source, a Ba source, a Ti source and a Zn source; the value ranges of x, y and z are respectively and independently 0-0.05, and the value ranges of x, y and z are not 0,0, w is more than or equal to 0.05,0, x + y + z + w is more than or equal to 0.15 at the same time;

and in an inert atmosphere, performing first sintering treatment on the precursor powder, mixing the precursor powder with a second organic carbon source for granulation, and performing second sintering treatment to obtain the iron-site-doped lithium manganese iron phosphate composite material.

5. The method for preparing the iron-site-doped lithium iron manganese phosphate composite material of claim 4, wherein the mixing and granulating step comprises: and preparing the product of the first sintering treatment and a second organic carbon source into mixed slurry, grinding and sieving the mixed slurry, and performing spray drying to obtain granulated powder.

6. The method of preparing an iron-site doped lithium iron manganese phosphate composite material of claim 4, wherein the conditions of the first sintering treatment comprise: sintering for 5-7 hours in inert atmosphere at 500-600 ℃.

7. The method for preparing an iron-site doped lithium iron manganese phosphate composite material according to claim 4, wherein the conditions of the second sintering treatment include: roasting for 8-10 hours in inert atmosphere at the temperature of 650-750 ℃.

8. The method for preparing the iron-site-doped lithium iron manganese phosphate composite material according to any one of claims 4 to 7, wherein the mass ratio of the raw material components to the first organic carbon source is 1: (0.1 to 0.2);

and/or the mass ratio of the product of the first sintering treatment to the second organic carbon source is 1: (0.03-0.08).

9. The method for preparing the iron-site doped lithium iron manganese phosphate composite material of claim 8, wherein the first organic carbon source and the second organic carbon source are each independently selected from at least one of citric acid, glucose, sucrose, soluble starch, salicylic acid, tartaric acid, oxalic acid, polysorbate, polyethylene glycol, sorbitan fatty acid, stearic acid, glycerol fatty acid ester, and amino acids;

and/or, the solvent comprises water and/or nitric acid;

and/or the lithium source comprises at least one of lithium carbonate, lithium hydroxide, lithium acetate, lithium oxalate and lithium chloride;

and/or the manganese source comprises at least one of manganous manganic oxide, manganese dioxide, manganese carbonate, manganese oxalate, manganese lactate, manganese chloride, manganese sulfate, manganese nitrate and manganese dioxide;

and/or the phosphorus source comprises at least one of phosphoric acid, diammonium phosphate and ammonium dihydrogen phosphate;

and/or the iron source comprises at least one of ferrous oxalate, ferric oxalate, ferrous acetate, ferric acetate, ferrous nitrate, ferric phosphate, ferric oxide, ferrous sulfate, ferric sulfate, ferrous chloride and ferric chloride;

and/or the magnesium source comprises at least one of magnesium nitrate, magnesium chloride, magnesium carbonate and magnesium sulfate;

and/or the vanadium source comprises NH ₄ VO ₃ At least one of vanadium nitrate and vanadium pentoxide;

and/or the aluminum source comprises at least one of aluminum nitrate, aluminum chloride, aluminum carbonate and aluminum sulfate;

and/or the Ca source comprises at least one of calcium hydroxide, calcium oxide and calcium salt;

and/or the Ba source comprises at least one of barium hydroxide, barium oxide and barium salt;

and/or the Ti source comprises at least one of titanium hydroxide, titanium oxide and titanium salt;

and/or the Zn source comprises at least one of zinc powder, zinc chloride, zinc sulfate and zinc gluconate.

10. A secondary battery is characterized by comprising a positive electrode, a negative electrode, a diaphragm and electrolyte; the positive electrode contains the iron site-doped lithium iron manganese phosphate composite material according to any one of claims 1 to 3 or the iron site-doped lithium iron manganese phosphate composite material prepared by the method according to any one of claims 4 to 9.

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