CN109309207B - Positive electrode active material, preparation method thereof, positive electrode and lithium ion battery - Google Patents
Positive electrode active material, preparation method thereof, positive electrode and lithium ion battery Download PDFInfo
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Abstract
本发明属于锂离子电池领域，尤其涉及一种正极活性物质及其制备方法、正极以及锂离子电池。该正极活性物质包括含磷酸锰铁锂的材料形成的核、位于核表面的第一外壳及位于第一外壳表面的第二外壳；所述第一外壳的材料为碳材料，所述第二外壳的材料为Mo2N，提高了材料的导电性和很好的解决了锰溶出的问题。
The invention belongs to the field of lithium ion batteries, and in particular relates to a positive electrode active material and a preparation method thereof, a positive electrode and a lithium ion battery. The positive active material includes a core formed of a material containing lithium iron manganese phosphate, a first shell located on the surface of the core, and a second shell located on the surface of the first shell; the material of the first shell is carbon material, and the second shell The material is Mo 2 N, which improves the conductivity of the material and solves the problem of manganese dissolution.
Description
Technical Field
The invention belongs to the field of lithium ion batteries, and particularly relates to a positive active material, a preparation method thereof, a lithium ion battery positive electrode containing the positive active material and a lithium ion battery.
Background
The lithium ion battery has the advantages of high voltage, large specific energy, good safety performance and the like, is more and more widely applied, and has higher and higher requirements particularly along with the popularization in electric vehicles, energy storage power stations and the like.
Lithium manganese iron phosphate is a high-capacity and high-safety battery positive electrode active material, but the lithium manganese iron phosphate exists: (1) poor conductivity; (2) in the charged state, elution of Fe and Mn from the positive electrode active material occurs, resulting in capacity fade; (3) excessive elution of metal ions also causes problems such as destruction of the material structure and deterioration of cycle performance, and has not been used in industry at a later date. To this end, is coated withIs one of the main improvement means at present. In the prior art, for example, in the patent with publication number CN104218218A, a lithium iron manganese phosphate lithium ion battery cathode material with a core-shell structure and a preparation method thereof are introduced, the material is composed of a core and a shell layer coated on the outer surface of the core, the chemical component of the core is Li (Fe)xMnyCoz)PO4The chemical composition of the shell layer is LiCoPO4X + y + z is 1, y is more than or equal to 0.5 and less than or equal to 0.7, z is more than or equal to 0.05 and less than or equal to 0.1, the cobalt-rich shell layer separates the Mn with strong solubility from the electrolyte to the maximum extent, and the dissolving degree of Mn is reduced in the using process, but LiCoPO4The charging and discharging platform voltage of the electrolyte is higher than the electrochemical window of the current commercial electrolyte, the doping capacity of the electrolyte cannot be exerted, so that the charging and discharging capacity of the whole material is influenced, and the LiCoPO4Has relatively poor conductivity and is coated with only LiCoPO4The conductivity of the material cannot be effectively improved. In patent publication No. CN106058225A, a core-shell structure LiMn is introduced1-xFexPO4The positive electrode material consists of a core and a shell, wherein the core is LiMn1-xFexPO4A nanoparticle; the shell is a mixture of carbon and a lithium-containing metal salt; the lithium-containing metal salt is lithium-containing metal phosphate and/or lithium-containing metal pyrophosphate; x is more than 0 and less than 0.5, and the mass of the carbon is LiMn1-xFexPO40.1-10% of the mass of the nano particles, but the core layer is LiMnxFe1-xPO4The shell layer is made of carbon and lithium-containing metal salt, the carbon is conductive, but other shell layer materials are all non-conductive, and the performance of the material is low. In patent publication No. CN104106161A, there is described an electrode active material comprising: a core formed of any one or a mixture selected from the group consisting of: lithium-containing transition metal oxides, carbon materials, lithium metal, and metal compounds; and a shell formed on a surface of the core and including lithium metal oxide particles and a polymer. The lithium metal oxide particles are particles of any one selected from the group consisting of: lithium iron phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium titanate, and vanadic acidLithium; the metal compound may be used in any form, such as pure metal, alloy, oxide, nitride, but it still suffers from manganese dissolution.
Disclosure of Invention
The invention aims to solve the problems of poor conductivity and manganese dissolution of the existing lithium ferric manganese phosphate material, and provides a positive active material with good rate capability and capable of effectively reducing the manganese dissolution phenomenon in the using process, a preparation method thereof, a positive electrode containing the positive active material and a lithium ion battery.
The first object of the invention is to provide a positive electrode active material, which comprises a core made of a material containing lithium manganese iron phosphate, a first shell positioned on the surface of the core and a second shell positioned on the surface of the first shell; the material of the first shell contains carbon, and the material of the second shell contains Mo2N。
A second object of the present invention is to provide a method for preparing a positive electrode active material, comprising: mixing the lithium iron manganese phosphate material with the carbon material attached to the surface with a molybdenum source, and sintering the mixture in the atmosphere of reducing gas and nitrogen or ammonia gas to obtain the positive active substance.
The third purpose of the invention is to provide a lithium ion battery positive electrode, which comprises the positive electrode active material or the positive electrode active material prepared by the preparation method of the positive electrode active material.
The fourth purpose of the invention is to provide a lithium ion battery, which comprises a positive electrode, a negative electrode and a diaphragm positioned between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode.
The positive active material provided by the invention is prepared by forming a carbon material and Mo on the surface of lithium manganese iron phosphate in situ2The shell material and the lithium manganese iron phosphate of the core material have good intermiscibility, a uniform coating layer can be formed on the surface of the shell material, the obtained positive active material of the core-shell structure is obtained, the coated carbon material can improve the conductivity of the material, and Mo is used as a material for improving the conductivity of the material2N has good conductivity and chemical corrosion resistance, and is coated with Mo2N not only can improve the conductivity of the material, but also can effectively reduce the used contentManganese dissolution in the process, in particular the invention, by connecting the core and the second shell Mo by an intermediate carbon layer2N, such that the core material and Mo2The connection tightness of N is good, the contact between manganese ions and electrolyte is better blocked, and the cycle performance of the battery is improved. And the anode active material is prepared by mixing the lithium iron manganese phosphate material with the carbon material attached to the surface and a molybdenum source and then sintering the mixture in the atmosphere of reducing gas and nitrogen or ammonia gas, wherein the molybdenum source is firstly reduced into simple substance molybdenum by the reducing gas such as hydrogen and then subjected to nitridation reaction to generate Mo2N, or molybdenum source directly reacts with ammonia gas to produce Mo2N, in the course of the reaction, not only N2Or NH3Participating in the reaction, and carburizing a small part of surface layer carbon of in-situ carbon on the surface of the lithium iron manganese phosphate-containing material so as to ensure that Mo2The N layer is in closer contact with the carbon layer, so that the contact between the electrolyte and the lithium manganese iron phosphate can be better isolated, the manganese dissolution phenomenon is reduced, the preparation cost is lower, and the industrial production is more facilitated.
Drawings
FIG. 1 is a charge-discharge graph of the battery S10 of example 1 of the present invention and the battery DS10 of comparative example 1.
FIG. 2 is a graph showing the cycle profiles of cell S10 of example 1 of the present invention and cell DS10 of comparative example 1.
Detailed Description
In order to make the technical problems, technical solutions and advantageous effects solved by the present invention more apparent, the present invention is further described in detail below with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The invention firstly provides a positive active material, which comprises a core formed by a material containing lithium manganese iron phosphate, a first shell positioned on the surface of the core and a second shell positioned on the surface of the first shell; the material of the first shell contains carbon, and the material of the second shell contains Mo2N, improves the conductivity of the material and solves the problem of manganese dissolution.
The material containing the lithium manganese iron phosphate is prepared by only containing phosphoric acidThe positive active material of the invention has high capacity and high safety of the lithium manganese iron phosphate, and simultaneously well solves the multiplying power and manganese dissolution problems of the lithium manganese iron phosphate, preferably, the material containing the lithium manganese iron phosphate is LiMn1-x-yFexMyPO4/C，0≤x<1，0≤y<1, M is at least one of Co, Ni, Al, Mg, Ga and 3d transition metal elements. More preferably, the material containing lithium manganese iron phosphate is LiMn1-x-yFexMyPO4and/C, wherein x is more than or equal to 0.1 and less than or equal to 0.2, y is more than or equal to 0 and less than or equal to 0.02, M is at least one of Co, Ni, Al, Mg, Ga and 3d transition group metal elements, and the material has high energy density and good conductivity.
Preferably, the carbon material is formed by in-situ cracking the carbon source on the surface of the core, the formed carbon layer has good compatibility with the core material, and the carbon material is uniformly coated on the surface of the core and has strong adhesive force, so that the subsequent Mo is convenient to obtain2In-situ preparation of N and benefit of Mo2And (4) tightly coating the N.
Preferably, the Mo is based on the mass of the positive electrode active material2The total content of N and the carbon material does not exceed 15wt.%, and the high capacity of the battery is further ensured.
Preferably, the Mo is based on the mass of the positive electrode active material2The content of N is 1-10 wt.%, and the content of the carbon material is 1-5 wt.%. More preferably, the Mo is based on the mass of the positive electrode active material2The content of N is 2-5 wt.%, and the content of the carbon material is 2-3 wt.%. Further improving the performance of the battery.
The application also provides a preparation method of the positive active material, which comprises the following steps: mixing the lithium iron manganese phosphate material with the carbon material attached to the surface with a molybdenum source, and sintering the mixture in the atmosphere of reducing gas and nitrogen or ammonia gas to obtain the positive active substance. Preferably, the sintering temperature is 500-800 ℃, the sintering time is 2-48 h, and the coating layer is further optimized. Preferably, the reducing gas is hydrogen. The reducing gas and nitrogen gas may be mixed, or phosphorus-containing gas with carbon material attached on surface thereof may be usedThe lithium iron manganese phosphate material with the carbon material attached to the surface is mixed with the molybdenum source and then sintered in the mixed atmosphere of the reducing gas and the nitrogen, and preferably, the volume ratio of the reducing gas to the nitrogen is 1/9-1. The molybdenum source is reduced into elemental molybdenum by a reducing gas such as hydrogen, and then subjected to a nitriding reaction to generate Mo2N, or molybdenum source directly reacts with ammonia gas to produce Mo2N, in the course of the reaction, not only N2Or NH3Participating in the reaction, and carburizing a small part of surface layer carbon of in-situ carbon on the surface of the lithium iron manganese phosphate-containing material so as to ensure that Mo2The N layer is in closer contact with the carbon layer, so that the contact between the electrolyte and the lithium manganese iron phosphate can be better isolated, and the manganese dissolution phenomenon is reduced.
The lithium iron manganese phosphate-containing material with the carbon material attached to the surface can be commercially available or can be prepared by itself, and the preparation method is not particularly limited in the application, for example, a sol-gel method, a solid phase method and the like can be adopted, and the solid phase method can be high-temperature sintering after ball milling of a lithium source, a manganese source, an iron source, a phosphorus source and a carbon source or high-temperature sintering after spray drying; the sintering temperature is generally 500-800 ℃. The detailed preparation process is not repeated herein.
The lithium source may be selected from LiH2PO4、Li2CO3、LiOH、CH3COOLi、LiF、LiBr、LiCl、LiI、Li2SO4、LiNO3、Li3PO4、Li2HPO4、Li2C2O4One or more of lithium tert-butoxide, lithium benzoate, lithium formate, lithium chromate, lithium citrate tetrahydrate, lithium aluminum tetrachloride and lithium tetrafluoroborate; said manganese source is selected from MnC2O4、Mn(OH)2、MnCO3、MnSO4、Mn(NO3)2、MnCl2Or one or more of manganese acetate; the iron source is selected from Fe3(PO4)2、FeC2O4、FeO、FeSO4One or more of ferric citrate, ferric stearate and ferric acetateSeveral kinds of the raw materials; the phosphorus source is selected from H3PO4、NH4H2PO4、(NH4)2HPO4、(NH4)3PO4、Li3PO4、Li2HPO4、LiH2PO4And P2O5One or more of the above; the molybdenum source is selected from Mo and MoO2、MoCl5、MoO3And (NH)4)6Mo7O24∙4H2One or more of O; the carbon source is selected from one or more of organic carbon sources such as sucrose, glucose, epoxy resin, polyvinyl alcohol and phenolic resin.
The application also provides a lithium ion battery anode, which comprises the anode active material or the anode active material prepared by the preparation method of the anode active material.
The positive electrode of the lithium ion battery generally includes a current collector and a positive electrode material attached to the current collector, the positive electrode material generally includes a positive electrode active material, a conductive agent and a binder, and the positive electrode active material is the positive electrode active material. The binder may be any binder known in the art, and for example, one or more of polyvinylidene fluoride, polytetrafluoroethylene, or styrene butadiene rubber may be used. The content of the binder is 0.1 to 15wt.%, preferably 1 to 7wt.% of the positive electrode active material. The conductive agent may be any conductive agent known in the art, and for example, one or more of graphite, carbon fiber, carbon black, metal powder, and fiber may be used. The content of the conductive agent is 0.1-20wt.%, preferably 2-10wt.% of the positive electrode active material.
The invention further provides a lithium ion battery, which comprises an anode, a cathode and a diaphragm positioned between the anode and the cathode, wherein the anode is the anode of the lithium ion battery.
The positive electrode can be prepared by various methods commonly used in the art, for example, by preparing a positive electrode active material, a binder and a conductive agent into a positive electrode material slurry using a solvent, which is well known to those skilled in the art, and can be applied according to the slurry of the positive electrode slurry to be preparedThe viscosity of the cloth and the operability requirements are flexibly adjusted. And then coating the prepared slurry of the positive electrode material on a positive electrode current collector, drying and tabletting, and cutting into pieces to obtain the positive electrode. The drying temperature is usually 60-140 ℃, and the drying time is usually 8-20 h. The solvent used in the positive electrode slurry may be any of various solvents known in the art, such as one or more selected from the group consisting of N-methylpyrrolidone (NMP), Dimethylformamide (DMF), Diethylformamide (DEF), Dimethylsulfoxide (DMSO), Tetrahydrofuran (THF), and water and alcohols. The solvent is used in an amount such that the slurry can be applied to the conductive substrate. Generally, the solvent is used in an amount such that the content of the positive electrode active material in the slurry is 40 to 90% by weight, preferably 50 to 85% by weight. The separator of the battery of the present invention has electrical insulation properties and liquid retention properties. The separator may be selected from various separators used in lithium ion secondary batteries well known to those skilled in the art, such as a polyolefin microporous membrane, a polyethylene felt, a glass fiber felt, or an ultrafine glass fiber paper. The electrolyte of the battery of the present invention is a nonaqueous electrolyte. The nonaqueous electrolytic solution is a solution of an electrolytic lithium salt in a nonaqueous solvent, and a conventional nonaqueous electrolytic solution known to those skilled in the art can be used. For example, the electrolyte lithium salt may be selected from lithium hexafluorophosphate (LiPF)6) Lithium perchlorate (LiClO)4) Lithium tetrafluoroborate (LiBF)4) Lithium hexafluoroarsenate (LiAsF)6) Lithium hexafluorosilicate (LiSiF)6) Lithium tetraphenylborate (LiB (C)6H5)4) Lithium chloride (LiCl), lithium bromide (LiBr), lithium chloroaluminate (LiAlCl)4) And fluoro-carbon lithium fluorosulfonate (LiC (SO)2CF3)3）、LiCH3SO3、LiN(SO2CF3)2One or more of them. The non-aqueous solvent can be selected from a chain acid ester and cyclic acid ester mixed solution, wherein the chain acid ester can be one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), Methyl Propyl Carbonate (MPC), dipropyl carbonate (DPC) and other chain organic esters containing fluorine, sulfur or unsaturated bonds. The cyclic acid ester can be ethyl carbonateAlkene Ester (EC), Propylene Carbonate (PC), Vinylene Carbonate (VC), gamma-butyrolactone (gamma-BL), sultone and other cyclic organic ester containing fluorine, sulfur or unsaturated bond. In the nonaqueous electrolytic solution, the concentration of the electrolytic lithium salt is generally 0.1 to 2 mol/liter, preferably 0.8 to 1.2 mol/liter. The negative electrode of the battery is not particularly limited, and is a negative electrode conventionally used in the prior art, and the preparation method of the negative electrode is well known to those skilled in the art and is not described herein again.
The method for manufacturing the battery of the present invention is well known to those skilled in the art, and generally, the method for manufacturing the battery includes placing a core into a battery case, adding an electrolyte, and then sealing to obtain the battery. The sealing method and the amount of the electrolyte are known to those skilled in the art.
The present invention will be described in further detail with reference to specific examples.
Example 1
Mixing LiH2PO4、MnCO3、FeC2O4Mixing with glucose according to the molar ratio of 1:0.8:0.2:0.092, and placing in a stirring ball mill for mixing and grinding for 8 h; putting the precursor prepared by fully mixing and grinding into a corundum crucible; putting the crucible into a tube furnace, introducing Ar gas, heating from room temperature at the heating rate of 5 ℃/min, heating to 700 ℃, roasting for 10h, and naturally cooling to room temperature to obtain LiMn0.8Fe0.2PO4And C, material. Mixing LiMn0.8Fe0.2PO4C and MoO3Mixing according to the mass ratio of 96:5.6, placing in a stirring ball mill, adding ethanol, and carrying out wet mixing and grinding for 6 hours; drying the precursor prepared by fully mixing and grinding, and then placing the precursor into a corundum crucible; the crucible is placed in a tube furnace, H containing 10% by volume of hydrogen is introduced2/N2Heating the mixed gas from room temperature at a heating rate of 5 ℃/min to 700 ℃, roasting for 15h, and naturally cooling to room temperature to obtain the Mo with the carbon content of 2wt.%, wherein the Mo is2LiMn with N content of 4 wt%0.8Fe0.2PO4/C/Mo2N composite sample S1.
Example 2
Mixing LiH2PO4、MnCO3、FeC2O4Mixing with glucose according to the molar ratio of 1:0.9:0.1:0.046, and placing in a stirring ball mill for mixing and grinding for 8 hours; putting the precursor prepared by fully mixing and grinding into a corundum crucible; putting the crucible into a tube furnace, introducing Ar gas, heating from room temperature at the heating rate of 5 ℃/min, heating to 700 ℃, roasting for 10h, and naturally cooling to room temperature to obtain LiMn0.9Fe0.1PO4And C, material. Mixing LiMn0.9Fe0.1PO4C and MoCl5Mixing according to the mass ratio of 95:13.3, adding water, stirring for 1h, and then carrying out spray drying to obtain powder, and placing the powder in a corundum crucible; putting the crucible into a tube furnace, introducing ammonia gas, heating from room temperature at a heating rate of 5 ℃/min, heating to 550 ℃, roasting for 20h, and naturally cooling to room temperature to obtain the Mo with the carbon content of 1wt.%, wherein the Mo is Mo2LiMn with N content of 5 wt%0.9Fe0.1PO4/C/Mo2N composite material, labeled S2.
Example 3
Mixing LiH2PO4、MnCO3、FeC2O4Mixing with glucose according to the molar ratio of 1:0.8:0.2:0.13, and placing the mixture in a stirring ball mill for mixing and grinding for 8 hours; putting the precursor prepared by fully mixing and grinding into a corundum crucible; putting the crucible into a tube furnace, introducing Ar gas, heating from room temperature at the heating rate of 5 ℃/min, heating to 700 ℃, roasting for 10h, and naturally cooling to room temperature to obtain LiMn0.8Fe0.2PO4And C, material. Mixing LiMn0.8Fe0.2PO4C and (NH)4)6Mo7O24∙4H2Mixing O according to the mass ratio of 97:5.1, adding water, stirring for 1h, then carrying out spray drying, and placing the obtained powder in a corundum crucible; putting the crucible into a tube furnace, introducing ammonia gas, heating from room temperature at a heating rate of 5 ℃/min, heating to 800 ℃, roasting for 2h, and naturally cooling to room temperature to obtain the product with the carbon content of 3wt.%, Mo2LiMn with N content of 3 wt%0.8Fe0.2PO4/C/Mo2N composite material, labeled S3.
Example 4
Mixing LiH2PO4、MnCO3、FeC2O4Mixing with glucose according to the molar ratio of 1:0.9:0.1:0.26, and placing in a stirring ball mill for mixing and grinding for 8 hours; putting the precursor prepared by fully mixing and grinding into a corundum crucible; putting the crucible into a tube furnace, introducing Ar gas, heating from room temperature at the heating rate of 5 ℃/min, heating to 700 ℃, roasting for 10h, and naturally cooling to room temperature to obtain LiMn0.9Fe0.1PO4And C, material. Mixing LiMn0.9Fe0.1PO4C and MoCl5Mixing according to the mass ratio of 90:26.5, adding water, stirring for 1h, and then carrying out spray drying to obtain powder, and placing the powder in a corundum crucible; putting the crucible into a tube furnace, introducing ammonia gas, heating from room temperature at a heating rate of 5 ℃/min, heating to 500 ℃, roasting for 48h, and naturally cooling to room temperature to obtain the Mo with the carbon content of 5wt.%, wherein the Mo is Mo2LiMn with N content of 10 wt%0.9Fe0.1PO4/C/Mo2N composite material, labeled S4.
Example 5
Mixing LiH2PO4、MnCO3、FeC2O4Mixing with glucose according to the molar ratio of 1:0.8:0.2:0.14, and placing in a stirring ball mill for mixing and grinding for 8 hours; putting the precursor prepared by fully mixing and grinding into a corundum crucible; putting the crucible into a tube furnace, introducing Ar gas, heating from room temperature at the heating rate of 5 ℃/min, heating to 700 ℃, roasting for 10h, and naturally cooling to room temperature to obtain LiMn0.8Fe0.2PO4And C, material. Mixing LiMn0.8Fe0.2PO4C and (NH)4)6Mo7O24∙4H2Mixing O according to the mass ratio of 95:8.6, adding water, stirring for 1h, and then carrying out spray drying to obtain powder and placing the powder in a corundum crucible; putting the crucible into a tube furnace, introducing ammonia gas, heating from room temperature at a heating rate of 5 ℃/min, heating to 700 ℃, roasting for 10h, and naturally cooling to room temperature to obtain the product with the carbon content of 3wt.%, Mo2Content of N is 5wt.%LiMn of0.8Fe0.2PO4/C/Mo2N composite material, labeled S5.
Example 6
Mixing LiH2PO4、MnCO3、FeC2O4Mixing with glucose according to the molar ratio of 1:0.8:0.2:0.58, and placing in a stirring ball mill for mixing and grinding for 8 hours; putting the precursor prepared by fully mixing and grinding into a corundum crucible; putting the crucible into a tube furnace, introducing Ar gas, heating from room temperature at the heating rate of 5 ℃/min, heating to 700 ℃, roasting for 10h, and naturally cooling to room temperature to obtain LiMn0.8Fe0.2PO4And C, material. Mixing LiMn0.8Fe0.2PO4C and MoO3Mixing according to the mass ratio of 96:21, placing in a stirring ball mill, adding ethanol, and carrying out wet mixing and grinding for 6 hours; drying the precursor prepared by fully mixing and grinding, and then placing the precursor into a corundum crucible; the crucible was placed in a tube furnace and 10% H was passed2/N2Heating the mixed gas from room temperature at a heating rate of 5 ℃/min to 800 ℃ for roasting for 10h, and naturally cooling to room temperature to obtain Mo with a carbon content of 10wt.%, wherein Mo is2LiMn with a N content of 15wt.%0.8Fe0.2PO4/C/Mo2N composite sample S6.
Examples 11 to 66
The battery is manufactured and tested according to the following modes, and the positive electrode sheets of the test batteries respectively adopt positive electrode materials according to the mass ratio (S1-S6 in sequence): acetylene black: PVDF = 85:10:5, and the mixture is pressed into tablets after being uniformly mixed, and the pole pieces are dried for more than 24 hours in vacuum at 120 ℃. 1mol/L LiPF with a metal lithium sheet as a cathode and a celgard2400 polypropylene porous membrane as a diaphragm6The mixed solution of Ethylene Carbonate (EC) and dimethyl carbonate (DMC) (volume ratio =1:1) is an electrolytic solution. The assembly of all cells was carried out in a glove box filled with argon, resulting in cell samples S10-S60 in sequence.
Comparative example 1
Mixing LiH2PO4、MnC2O4、FeC2O4Mixing with glucose at a molar ratio of 1:0.8:0.2:0.28, and standingMixing and grinding for 8 hours in a stirring ball mill; putting the precursor prepared by fully mixing and grinding into a corundum crucible; putting the crucible into a tube furnace, introducing Ar gas, heating from room temperature at the heating rate of 5 ℃/min, heating to 700 ℃, roasting for 10h, and naturally cooling to room temperature to obtain LiMn with the carbon content of 6 wt%0.8Fe0.2PO4the/C composite material is marked as DS 1.
Comparative example 2
Mixing LiH2PO4、MnCO3And FeC2O4Mixing according to the molar ratio of 1:0.8:0.2, and placing the mixture in a stirring ball mill for mixing and grinding for 8 hours; putting the precursor prepared by fully mixing and grinding into a corundum crucible; putting the crucible into a tube furnace, introducing Ar gas, heating from room temperature at the heating rate of 5 ℃/min, heating to 700 ℃, roasting for 10h, and naturally cooling to room temperature to obtain LiMn0.8Fe0.2PO4A material. Mixing LiMn0.8Fe0.2PO4And MoCl5Mixing according to the mass ratio of 94:15.9, adding water, stirring for 1h, and then carrying out spray drying to obtain powder, and placing the powder in a corundum crucible; putting the crucible into a tube furnace, introducing ammonia gas, heating from room temperature at a heating rate of 5 ℃/min to 700 ℃, roasting for 10h, and naturally cooling to room temperature to obtain Mo2LiMn with a N content of 6wt.%0.8Fe0.2PO4/ Mo2N composite material, labeled DS 2.
Comparative examples 11 to 22
The battery is manufactured and tested according to the following modes, and the positive plates of the test batteries respectively adopt positive materials (DS 1-DS2 in sequence) according to mass ratios: acetylene black: PVDF = 85:10:5, and the mixture is pressed into tablets after being uniformly mixed, and the pole pieces are dried for more than 24 hours in vacuum at 120 ℃. 1mol/L LiPF with a metal lithium sheet as a cathode and a celgard2400 polypropylene porous membrane as a diaphragm6The mixed solution of Ethylene Carbonate (EC) and dimethyl carbonate (DMC) (volume ratio =1:1) is an electrolytic solution. The assembly of all cells was carried out in a glove box filled with argon, which in turn yielded cell samples DS10-DS 20.
And (3) performance testing:
1. specific capacity of charge and discharge
The battery is set to a charging state, namely the lithium is removed from the working electrode, and the charging current density is 0.1mA/cm2And stopping operation when the charging is carried out to the cut-off voltage of 4.3V, and calculating the first charging specific capacity.
First-time lithium removal specific capacity (mAh/g) = first-time lithium removal capacity/mass of active material
After the first lithium removal, the battery is set to be in a discharge state, namely the working electrode is embedded with lithium, and the discharge current density is 0.1mA/cm2And finishing discharging when the discharging voltage reaches 2.5V, and calculating the first discharging specific capacity.
First lithium intercalation specific capacity (mAh/g) = first lithium intercalation capacity/mass of active material
The test results are shown in fig. 1 and table 1.
2. Cycle performance
And (3) carrying out constant current charging on the battery by using a constant current of 0.1mA, charging to a cut-off voltage, carrying out constant current discharging on the battery by using a constant current of 0.1mA, discharging to the cut-off voltage, standing for 10 minutes, repeating the steps, carrying out continuous charging and discharging tests to obtain the battery capacity after 500 cycles of the battery, and calculating the discharging capacity retention rate of the battery after 500 cycles.
Discharge capacity retention rate =500 cycles after discharge capacity/first discharge capacity × 100%
The test results are shown in fig. 2 and table 1.
Fig. 1 is a charge and discharge curve at a charge and discharge rate of 0.1C for the battery sample S10 produced in example 1 and the battery sample DS10 produced in comparative example 1. It was found that the first charge capacity of S10 was 155.0mAh/g, the first discharge capacity was 150.7mAh/g, the first charge capacity of DS10 was 154.3mAh/g, and the first discharge capacity was 149.5 mAh/g. The capacity and voltage plateau of the S10 and DS10 batteries are not obviously different, which indicates that Mo2The coating of N can effectively improve the conductivity of the material like the coating of carbon.
FIG. 2 is a cycle life curve of the battery sample S10 produced in example 1 and the battery sample DS10 produced in comparative example 1, and it can be seen that the discharge capacity after 500 cycles of the S10 battery was 146.3mAh/g, the discharge capacityThe retention rate is 97.1%, the discharge capacity of the DS10 battery after 500 cycles is 136.4mAh/g, the retention rate of the discharge capacity is 91.2%, and the cycle result shows that LiMn0.8Fe0.2PO4/C/Mo2The N composite material has a specific LiMn0.8Fe0.2PO4The more excellent charge-discharge cycle stability of the/C composite material shows that Mo2The coating of N can effectively prevent the metal ions from dissolving out.
TABLE 1
As can be seen from the test results of table 1, the positive active material of the present invention has high capacity and good cycle performance.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.
Claims (13)
1. The positive active material is characterized by comprising a core made of a material containing lithium manganese iron phosphate, a first shell positioned on the surface of the core and a second shell positioned on the surface of the first shell; the material of the first shell contains carbon, and the material of the second shell contains Mo2N；
Based on the mass of the positive electrode active material, the Mo2The content of N is 2-5 wt.%, and the content of the carbon material is 2-3 wt.%.
2. The positive electrode active material according to claim 1, wherein the material containing lithium iron manganese phosphate is LiMn1-x-yFexMyPO4/C，0≤x<1，0≤y<1, M is at least one of Co, Ni, Al, Mg, Ga and 3d transition metal elements.
3. The positive electrode active material according to claim 1, wherein the carbon material is a carbon material formed by in-situ cleavage of a carbon source on a core surface.
4. The positive electrode active material according to claim 1, wherein the Mo is contained in an amount based on the mass of the positive electrode active material2The total content of N and carbon material does not exceed 15 wt.%.
5. The positive electrode active material according to claim 1, wherein the Mo is contained in an amount based on the mass of the positive electrode active material2The content of N is 1-10 wt.%, and the content of the carbon material is 1-5 wt.%.
6. A method for producing a positive electrode active material, comprising: the positive electrode active material according to any one of claims 1 to 5 is obtained by mixing a lithium iron manganese phosphate-containing material having a carbon material attached to the surface thereof with a molybdenum source, and then sintering the mixture in an atmosphere of a reducing gas and nitrogen gas or an ammonia gas.
7. The method for preparing a positive electrode active material according to claim 6, wherein the lithium iron manganese phosphate-containing material having the carbon material attached to the surface thereof is prepared from a lithium source, a manganese source, an iron source, a phosphorus source, and a carbon source.
8. The method for producing a positive electrode active material according to claim 7, wherein the lithium source is LiH2PO4、Li2CO3、LiOH、CH3COOLi、LiF、LiBr、LiCl、LiI、Li2SO4、LiNO3、Li3PO4、Li2HPO4、Li2C2O4One or more of lithium tert-butoxide, lithium benzoate, lithium formate, lithium chromate, lithium citrate tetrahydrate, lithium aluminum tetrachloride and lithium tetrafluoroborate; said manganese source is selected from MnC2O4、Mn(OH)2、MnCO3、MnSO4、Mn(NO3)2、MnCl2Or one or more of manganese acetate; the above-mentionedThe iron source is selected from Fe3(PO4)2、FeC2O4、FeO、FeSO4One or more of ferric citrate, ferric stearate and ferric acetate; the phosphorus source is selected from H3PO4、NH4H2PO4、(NH4)2HPO4、(NH4)3PO4、Li3PO4、Li2HPO4、LiH2PO4And P2O5One or more of the above; the molybdenum source is selected from Mo and MoO2、MoCl5、MoO3And (NH)4)6Mo7O24·4H2One or more of O; the carbon source is selected from one or more of sucrose, glucose, epoxy resin, polyvinyl alcohol and phenolic resin.
9. The method for producing a positive electrode active material according to claim 6, wherein the sintering temperature is 500 to 800 ℃ and the sintering time is 2 to 48 hours.
10. The method for producing a positive electrode active material according to claim 6, wherein the reducing gas is hydrogen gas.
11. The method for producing a positive electrode active material according to claim 6, wherein the volume ratio of the reducing gas to the nitrogen gas is 1/9-1.
12. A positive electrode for a lithium ion battery, comprising the positive electrode active material according to any one of claims 1 to 5 or the positive electrode active material produced by the method for producing a positive electrode active material according to any one of claims 6 to 11.
13. A lithium ion battery comprising a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode according to claim 12.
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