CN113555557A - Lithium iron phosphate anode material, preparation method and application thereof - Google Patents
Lithium iron phosphate anode material, preparation method and application thereof Download PDFInfo
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- CN113555557A CN113555557A CN202110764226.9A CN202110764226A CN113555557A CN 113555557 A CN113555557 A CN 113555557A CN 202110764226 A CN202110764226 A CN 202110764226A CN 113555557 A CN113555557 A CN 113555557A
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- iron phosphate
- lithium iron
- positive electrode
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- GELKBWJHTRAYNV-UHFFFAOYSA-K lithium iron phosphate Chemical compound [Li+].[Fe+2].[O-]P([O-])([O-])=O GELKBWJHTRAYNV-UHFFFAOYSA-K 0.000 title claims abstract description 104
- 239000010405 anode material Substances 0.000 title claims abstract description 23
- 238000002360 preparation method Methods 0.000 title claims abstract description 16
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims abstract description 54
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 39
- 239000010406 cathode material Substances 0.000 claims abstract description 27
- 229910001416 lithium ion Inorganic materials 0.000 claims abstract description 19
- HBBGRARXTFLTSG-UHFFFAOYSA-N Lithium ion Chemical compound [Li+] HBBGRARXTFLTSG-UHFFFAOYSA-N 0.000 claims abstract description 18
- 239000002344 surface layer Substances 0.000 claims abstract description 18
- 239000000243 solution Substances 0.000 claims description 185
- 238000006243 chemical reaction Methods 0.000 claims description 89
- 239000002243 precursor Substances 0.000 claims description 82
- NBIIXXVUZAFLBC-UHFFFAOYSA-N Phosphoric acid Chemical compound OP(O)(O)=O NBIIXXVUZAFLBC-UHFFFAOYSA-N 0.000 claims description 72
- WMFOQBRAJBCJND-UHFFFAOYSA-M Lithium hydroxide Chemical compound [Li+].[OH-] WMFOQBRAJBCJND-UHFFFAOYSA-M 0.000 claims description 71
- 239000007864 aqueous solution Substances 0.000 claims description 56
- 239000013078 crystal Substances 0.000 claims description 54
- 229910019142 PO4 Inorganic materials 0.000 claims description 49
- 239000007774 positive electrode material Substances 0.000 claims description 42
- 229910052493 LiFePO4 Inorganic materials 0.000 claims description 40
- 229910000155 iron(II) phosphate Inorganic materials 0.000 claims description 37
- 229910000147 aluminium phosphate Inorganic materials 0.000 claims description 36
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 31
- 238000002156 mixing Methods 0.000 claims description 30
- 229910001386 lithium phosphate Inorganic materials 0.000 claims description 29
- MUBZPKHOEPUJKR-UHFFFAOYSA-N Oxalic acid Chemical compound OC(=O)C(O)=O MUBZPKHOEPUJKR-UHFFFAOYSA-N 0.000 claims description 27
- KRKNYBCHXYNGOX-UHFFFAOYSA-N citric acid Chemical compound OC(=O)CC(O)(C(O)=O)CC(O)=O KRKNYBCHXYNGOX-UHFFFAOYSA-N 0.000 claims description 27
- 239000008139 complexing agent Substances 0.000 claims description 27
- 239000012046 mixed solvent Substances 0.000 claims description 25
- 239000002253 acid Substances 0.000 claims description 24
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 claims description 17
- 238000000034 method Methods 0.000 claims description 17
- 239000010452 phosphate Substances 0.000 claims description 16
- NBIIXXVUZAFLBC-UHFFFAOYSA-K phosphate Chemical compound [O-]P([O-])([O-])=O NBIIXXVUZAFLBC-UHFFFAOYSA-K 0.000 claims description 15
- 150000001875 compounds Chemical class 0.000 claims description 13
- BAUYGSIQEAFULO-UHFFFAOYSA-L iron(2+) sulfate (anhydrous) Chemical compound [Fe+2].[O-]S([O-])(=O)=O BAUYGSIQEAFULO-UHFFFAOYSA-L 0.000 claims description 13
- 239000002245 particle Substances 0.000 claims description 13
- CWYNVVGOOAEACU-UHFFFAOYSA-N Fe2+ Chemical class [Fe+2] CWYNVVGOOAEACU-UHFFFAOYSA-N 0.000 claims description 12
- CIWBSHSKHKDKBQ-JLAZNSOCSA-N Ascorbic acid Chemical compound OC[C@H](O)[C@H]1OC(=O)C(O)=C1O CIWBSHSKHKDKBQ-JLAZNSOCSA-N 0.000 claims description 10
- 235000015165 citric acid Nutrition 0.000 claims description 9
- 239000010410 layer Substances 0.000 claims description 9
- 239000003960 organic solvent Substances 0.000 claims description 9
- 235000006408 oxalic acid Nutrition 0.000 claims description 9
- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 claims description 8
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 8
- 229910052744 lithium Inorganic materials 0.000 claims description 8
- -1 phosphate radical ions Chemical class 0.000 claims description 8
- 239000002041 carbon nanotube Substances 0.000 claims description 7
- 229910021393 carbon nanotube Inorganic materials 0.000 claims description 7
- 229910021389 graphene Inorganic materials 0.000 claims description 7
- 229910000904 FeC2O4 Inorganic materials 0.000 claims description 6
- 239000006185 dispersion Substances 0.000 claims description 6
- 239000011668 ascorbic acid Substances 0.000 claims description 5
- 235000010323 ascorbic acid Nutrition 0.000 claims description 5
- 229960005070 ascorbic acid Drugs 0.000 claims description 5
- 229910000359 iron(II) sulfate Inorganic materials 0.000 claims description 5
- 229910021577 Iron(II) chloride Inorganic materials 0.000 claims description 4
- 229910003481 amorphous carbon Inorganic materials 0.000 claims description 4
- 239000011230 binding agent Substances 0.000 claims description 4
- 239000006258 conductive agent Substances 0.000 claims description 4
- 229910000397 disodium phosphate Inorganic materials 0.000 claims description 4
- 150000002500 ions Chemical class 0.000 claims description 4
- 239000002904 solvent Substances 0.000 claims description 4
- 125000003158 alcohol group Chemical group 0.000 claims description 3
- LFVGISIMTYGQHF-UHFFFAOYSA-N ammonium dihydrogen phosphate Chemical group [NH4+].OP(O)([O-])=O LFVGISIMTYGQHF-UHFFFAOYSA-N 0.000 claims description 3
- 229910000388 diammonium phosphate Inorganic materials 0.000 claims description 3
- BNIILDVGGAEEIG-UHFFFAOYSA-L disodium hydrogen phosphate Chemical compound [Na+].[Na+].OP([O-])([O-])=O BNIILDVGGAEEIG-UHFFFAOYSA-L 0.000 claims description 3
- NMCUIPGRVMDVDB-UHFFFAOYSA-L iron dichloride Chemical group Cl[Fe]Cl NMCUIPGRVMDVDB-UHFFFAOYSA-L 0.000 claims description 3
- 238000004519 manufacturing process Methods 0.000 claims description 2
- 238000005253 cladding Methods 0.000 claims 2
- 229940085991 phosphate ion Drugs 0.000 claims 1
- 238000003756 stirring Methods 0.000 description 40
- 230000001276 controlling effect Effects 0.000 description 30
- QTBSBXVTEAMEQO-UHFFFAOYSA-N Acetic acid Chemical compound CC(O)=O QTBSBXVTEAMEQO-UHFFFAOYSA-N 0.000 description 21
- LYCAIKOWRPUZTN-UHFFFAOYSA-N Ethylene glycol Chemical compound OCCO LYCAIKOWRPUZTN-UHFFFAOYSA-N 0.000 description 21
- 239000008367 deionised water Substances 0.000 description 18
- 229910021641 deionized water Inorganic materials 0.000 description 18
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 16
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 16
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- 229910052757 nitrogen Inorganic materials 0.000 description 8
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- 229910017677 NH4H2 Inorganic materials 0.000 description 4
- PQLVXDKIJBQVDF-UHFFFAOYSA-N acetic acid;hydrate Chemical compound O.CC(O)=O PQLVXDKIJBQVDF-UHFFFAOYSA-N 0.000 description 4
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
- H01M4/624—Electric conductive fillers
- H01M4/625—Carbon or graphite
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/136—Electrodes based on inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
- H01M4/5825—Oxygenated metallic salts or polyanionic structures, e.g. borates, phosphates, silicates, olivines
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- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/021—Physical characteristics, e.g. porosity, surface area
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
- H01M2004/028—Positive electrodes
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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Abstract
The invention provides a lithium iron phosphate anode material, a preparation method and application thereof. The lithium iron phosphate anode material comprises spherical lithium iron phosphate and carbon compounded on the surface of the spherical lithium iron phosphate, wherein the spherical lithium iron phosphate is provided with a core and a surface layer positioned on the periphery of the core, the carbon is coated on the surface layer, and the porosity of the core is greater than that of the surface layer. The lithium iron phosphate cathode material provided by the invention has good dynamic performance (rate capability) and discharge capacity after being applied to a lithium ion battery, has excellent electrochemical performance, and is remarkable in capacity, service life and safety.
Description
Technical Field
The invention relates to the technical field of lithium ion batteries, in particular to a lithium iron phosphate positive electrode material, and a preparation method and application thereof.
Background
Lithium iron phosphate (LiFePO)4) Because of its high safety performance and considerable electrochemical performance, it is one of the most successful commercial anode materials in the field of lithium ion batteries at present. However, its lower energy density, poorer kineticThe commercial development and progress of lithium iron phosphate are limited by the defects of mechanical property, lithium ion diffusion capacity and the like. The anode material is an important component of the lithium ion battery, and has the performance of capacity, service life, cost, safety and the like of the lithium ion battery. Therefore, the development of the lithium iron phosphate positive electrode material with high reversible discharge capacity, excellent cycle performance and high thermal stability has important practical significance.
Disclosure of Invention
The invention mainly aims to provide a lithium iron phosphate positive electrode material, a preparation method and application thereof, and aims to solve the defects of lithium iron phosphate in the prior art in the aspects of capacity, service life, safety and the like.
In order to achieve the above object, according to one aspect of the present invention, there is provided a lithium iron phosphate positive electrode material, including spherical lithium iron phosphate and carbon compounded on a surface of the spherical lithium iron phosphate, where the spherical lithium iron phosphate has a core and a surface layer located at a periphery of the core, the carbon is coated on the surface layer, and a porosity of the core is greater than a porosity of the surface layer.
Further, the particle size of the spherical lithium iron phosphate is 4-6 μm; the particle size of the inner core is 1-3 μm.
Further, the porosity of the inner core is 60-90%; the surface layer has a porosity of 0 to 20%.
Further, the carbon is selected from one or more of graphene, carbon nanotubes, conductive carbon black and amorphous carbon; the content of carbon is 1.5-2.5% of the weight of the lithium iron phosphate anode material.
According to another aspect of the present invention, there is also provided a preparation method of a lithium iron phosphate positive electrode material, comprising the following steps: preparing a mixed solvent of water and an organic solvent, adding a binary solution, a complexing agent solution and an acid solution into the mixed solvent, and carrying out a first-stage reaction under the condition that the pH value is 1.8-2.4 to form an intermediate reaction solution; adjusting the pH value of the intermediate reaction solution to 2.6-3.2, and carrying out the second stage reaction to form Fe3(PO4)2A precursor; wherein the binary solution is a solution of soluble ferrous salt and soluble compound containing phosphate radical ions; mixing Fe3(PO4)2Precursor dispersion to Li3PO4Crystal nucleus growth is carried out in the crystal solution to obtain LiFePO4A precursor; mixing LiFePO4And mixing the precursor with a carbon source, and then roasting in an inert atmosphere to obtain the lithium iron phosphate anode material.
Furthermore, the organic solvent in the mixed solvent is an alcohol solvent, and the volume ratio of water to the organic solvent in the mixed solvent is (1-3) to (1-3).
Further, the soluble ferrous salt is selected from FeCl2、FeC2O4、(CH3COO)2Fe、FeSO4One or more of; the soluble compound containing phosphate ions is selected from phosphoric acid and/or soluble phosphate, and the soluble phosphate is selected from NH4H2PO4，Na2HPO4，(NH4)2HPO4One or more of; the binary solution is a soluble ferrous salt and a water solution of soluble compound containing phosphate radical ions, wherein Fe in the soluble ferrous salt2+With PO in soluble compounds containing phosphate ions4 3+The molar ratio of (2.85-3.15) to (2), and the total molar concentration of the soluble ferrous salt and the phosphate radical-containing ions is 1-3 mol/L; in the complexing agent solution, the complexing agent is selected from one or more of citric acid, oxalic acid and ascorbic acid.
Further, the crystal nucleus growing step includes: adding phosphoric acid solution into lithium hydroxide solution for reaction to form Li3PO4A crystal solution; mixing Fe3(PO4)2Precursor dispersion to Li3PO4Crystal nucleus growth is carried out in the crystal solution to obtain LiFePO4A precursor; the phosphoric acid solution is a phosphoric acid water solution, and the molar concentration of the phosphoric acid water solution is 1.0-2.5 mol/L; the lithium hydroxide solution is a lithium hydroxide aqueous solution, and the molar concentration of the lithium hydroxide aqueous solution is 1.0-2.5 mol/L; li in lithium hydroxide solution+With PO in phosphoric acid solution4 3-The molar ratio of (2.85-3.15): 1.
According to another aspect of the invention, a cathode material is also provided, which is the lithium iron phosphate cathode material or the lithium iron phosphate cathode material prepared by the preparation method.
According to another aspect of the present invention, a lithium ion battery is further provided, which includes a positive electrode, where the positive electrode includes a positive electrode current collector and a positive electrode active layer located on the surface of the positive electrode current collector, and the positive electrode active layer includes a positive electrode material, a conductive agent and a binder, where the positive electrode material is the above-mentioned lithium iron phosphate positive electrode material, or is the lithium iron phosphate positive electrode material prepared by the above-mentioned preparation method.
The lithium iron phosphate anode material provided by the invention comprises spherical lithium iron phosphate and carbon compounded on the surface of the spherical lithium iron phosphate, wherein the spherical lithium iron phosphate is provided with a core and a surface layer positioned on the periphery of the core, and the porosity of the core is greater than that of the surface layer. The porosity of the spherical lithium iron phosphate core is greater than that of the surface layer, and the spherical lithium iron phosphate core is equivalent to a hollow structure with a loose internal structure and a compact shell structure. The structure can obviously improve the specific surface area of the material, and further effectively improve the lithium storage efficiency of the anode. Meanwhile, the carbon compounded on the surface can also synergistically act with spherical lithium iron phosphate with a hollow structure to improve the conductivity of the material, so that the diffusion capacity of lithium ions is improved, and the lithium iron phosphate anode material has good dynamic performance (rate capability) and discharge gram capacity and excellent electrochemical performance after being applied to a lithium ion battery due to the two reasons. Meanwhile, the lithium iron phosphate cathode material has good thermal stability. The above aspects make the material show outstanding capacity, service life and safety after being applied to a battery.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the invention and not to limit the invention. In the drawings:
fig. 1 shows an XRD structure diagram corresponding to the lithium iron phosphate cathode material prepared in example 1;
fig. 2 shows a first charge-discharge curve diagram of a button cell corresponding to the lithium iron phosphate positive electrode material prepared in example 1;
fig. 3 shows a rate performance curve of a soft-package cell corresponding to the lithium iron phosphate positive electrode material prepared in example 1;
fig. 4 shows GITT curves of button cells corresponding to the lithium iron phosphate positive electrode material prepared in example 1;
fig. 5 shows a 45 ℃ circulation capacity retention rate curve of a soft-package battery cell corresponding to the lithium iron phosphate cathode material prepared in example 1;
fig. 6 shows a DSC thermal analysis graph corresponding to the lithium iron phosphate positive electrode material prepared in example 1;
fig. 7 shows a first charge-discharge curve diagram of a button cell corresponding to the lithium iron phosphate cathode material prepared in comparative example 1.
Detailed Description
It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict. The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings.
As described in the background section, the lithium iron phosphate in the prior art has shortcomings in capacity, lifetime, safety, and the like. In order to solve the problem, the invention provides a lithium iron phosphate positive electrode material which is characterized by comprising spherical lithium iron phosphate and carbon compounded on the surface of the spherical lithium iron phosphate, wherein the spherical lithium iron phosphate is provided with a core and a surface layer positioned on the periphery of the core, the carbon is coated on the surface layer, and the porosity of the core is greater than that of the surface layer.
The porosity of the spherical lithium iron phosphate core is greater than that of the surface layer, and the spherical lithium iron phosphate core is equivalent to a hollow structure with a loose internal structure and a compact shell structure. The structure can obviously improve the specific surface area of the material, further effectively improve the lithium storage efficiency of the anode, specifically, more electrolytes can be effectively stored, the long-term cycle performance in the charging and discharging process is improved, more storage accommodating spaces can be provided under the overcharge condition of the lithium ion battery, and the structural stability and the safety performance are improved. And the presence of special hollow structures, Li+The migration path distance and the diffusion migration resistance are reduced, the smooth de-intercalation of Li < + > is ensured by the increase of surface contact active sites, and the migration capability of ions is improved. Correspondingly, the first charge-discharge efficiency, coulombic efficiency, rate capability and long-term performance of the batteryThe cycle performance is improved. Meanwhile, the carbon compounded on the surface can also improve the conductivity of the material by the synergistic effect of the carbon and the spherical lithium iron phosphate with a hollow structure, and the lithium ion diffusion capacity is improved. Due to the reasons, the lithium iron phosphate cathode material has good dynamic performance (rate capability) and discharge gram capacity and excellent electrochemical performance after being applied to the lithium ion battery. Meanwhile, the lithium iron phosphate cathode material has good thermal stability. The above aspects make the material show outstanding capacity, service life and safety after being applied to a battery.
In order to further improve various performances of the cathode material and fully exert the advantages brought by the hollow structure, in a preferred embodiment, the particle size of the spherical lithium iron phosphate is 4-6 μm; preferably, the particle size of the inner core is 1-3 μm. More preferably, the porosity of the inner core is 60-90%; the surface layer has a porosity of 0 to 20%.
The carbon compounded on the surface of the spherical lithium iron phosphate has good conductivity, and exemplarily, the carbon includes but is not limited to one or more of graphene, carbon nanotubes, conductive carbon black and amorphous carbon; preferably, the content of carbon is 1.5-2.5% of the weight of the lithium iron phosphate positive electrode material. The compounding amount of carbon is controlled within the range, so that the conductivity of the anode material can be further improved, and meanwhile, the anode material is favorably ensured to have higher energy density.
According to another aspect of the present invention, there is also provided a preparation method of a lithium iron phosphate positive electrode material, comprising the following steps: preparing a mixed solvent of water and an organic solvent, adding a binary solution, a complexing agent solution and an acid solution into the mixed solvent, and carrying out a first-stage reaction under the condition that the pH value is 1.8-2.4 to form an intermediate reaction solution; adjusting the pH value of the intermediate reaction solution to 2.6-3.2, and carrying out the second stage reaction to form Fe3(PO4)2A precursor; wherein the binary solution is a solution of soluble ferrous salt and soluble compound containing phosphate radical ions; mixing Fe3(PO4)2Precursor dispersion to Li3PO4Crystal nucleus growth is carried out in the crystal solution to obtain LiFePO4A precursor; mixing LiFePO4And mixing the precursor with a carbon source, and then roasting in an inert atmosphere to obtain the lithium iron phosphate anode material.
In the preparation method, firstly, a binary solution formed by soluble ferrous salt and soluble compound containing phosphate radical ions is used for reaction in a mixed solvent to prepare Fe3(PO4)2And (3) precursor. On one hand, the process takes the mixed solvent as the reaction solution, can provide a relatively mild environment for the production of the precursor, and is beneficial to the growth nucleation and the oriented growth of the crystal grain seeds; the amorphous precursor is separated out in the solution to generate tiny particles, the initially formed crystal particles are increased, the tiny particles take micelles as crystal nuclei to reduce the surface tension effect and the free energy, the particles are gradually gathered and are accumulated and grown at the periphery, and finally a spherical structure is formed. On the other hand, the reaction of two stages is carried out under the respective specific pH environments, so that Fe3(PO4)2The primary particle structures of the inner core and the outer shell of the precursor are different, specifically, the outer shell is agglomerated and compact, the sintering temperature is high, the structure formed inside is loose, and more pores are formed. Subsequent further growth of LiFePO on the basis of the primary particles4After the precursor, this structure can still be maintained, so that a special hollow structure with an internal porosity greater than the external porosity can be formed during the final firing. In addition, in the reaction process of the two stages, the pH value can be adjusted to a target range by utilizing the acid solution, and the addition of the complexing agent solution is more beneficial to Fe3(PO4)2The growth of the precursor further improves the morphology of the final lithium iron phosphate, so that the lithium iron phosphate has a complete spherical structure and has a better promotion effect on various performances of the anode material.
In the formation of Fe3(PO4)2After the precursor, it is added to Li3PO4Crystal nucleus growth is carried out in the crystal solution to obtain LiFePO4And (3) precursor. In the roasting stage LiFePO is used4The precursor is mixed with a carbon source, so that a carbon material can be compounded on the surface of the spherical lithium iron phosphate, and the conductivity of the anode material is further improved.
By adopting the preparation method, the spherical lithium iron phosphate with the hollow structure can be formed, and the carbon material is compounded on the surface of the spherical lithium iron phosphate, as mentioned above, the structure and the composition enable the positive electrode material to have better first charge-discharge efficiency, coulombic efficiency, rate capability, long-term cycle performance and the like of the battery, and the positive electrode material is remarkable in capacity, service life, safety and the like.
To further increase Fe3(PO4)2The stability of precursor reaction and growth, in a preferred embodiment, the organic solvent in the mixed solvent is an alcohol solvent, preferably an alcohol solvent polyol, more preferably one or more selected from ethylene glycol, liquid polyethylene glycol (such as polyethylene glycol 200, polyethylene glycol 400, etc.), glycerol; preferably, the volume ratio of the water to the organic solvent in the mixed solvent is (1-3): 1-3, such as 1:1, 1:2, 1:3, 1:1, 2:1, 3:1, etc. The adoption of the mixed solvent is beneficial to further improving Fe3(PO4)2The reaction and growth stability of the precursor, the crystal structure of the precursor is more complete, and the subsequent LiFePO treatment4After the precursor grows and is roasted, the obtained lithium iron phosphate hollow spherical structure is more complete, and the improvement of the comprehensive performance of the anode material is facilitated.
In a preferred embodiment, the soluble ferrous salts include, but are not limited to FeCl2、FeC2O4、(CH3COO)2Fe、FeSO4One or more of; preferably, the soluble compound containing phosphate ions is selected from phosphoric acid and/or a soluble phosphate, more preferably the soluble phosphate is selected from NH4H2PO4，Na2HPO4，(NH4)2HPO4One or more of (a). The soluble ferrous salts and the soluble compound containing phosphate ions can react and grow more stably in the mixed solvent. Preferably, the binary solution is an aqueous solution of a soluble ferrous salt and a soluble compound containing phosphate ions, wherein the soluble ferrous salt contains Fe2+With PO in soluble compounds containing phosphate ions4 3+The molar ratio of (2.85-3.15) to (2), and the total molar concentration of the soluble ferrous salt and the phosphate radical-containing ions is 1-3 mol/L.
Addition of complexing agent solution is beneficial to Fe3(PO4)2Precursor crystal growth, to further enhance this effect, in a preferred embodiment,
in the complexing agent solution, the complexing agent is selected from one or more of citric acid, oxalic acid and ascorbic acid; preferably, the complexing agent solution is an aqueous solution of the complexing agent, and the molar concentration of the complexing agent solution is 3-5 mol/L; in addition, the pH value of the reaction system can be timely adjusted through an acid solution, more preferably, the acid in the acid solution is selected from one or more of carbonic acid, phosphoric acid and acetic acid, and preferably, the acid solution is an aqueous solution of the acid, and the molar concentration of the aqueous solution is 0.1-0.5 mol/L. In order to stabilize the reaction, in a preferred embodiment, when the mixed solvent is added, the adding amount of the binary solution is 80-90% of the volume of the mixed solvent, the adding amount of the complexing agent solution is 10-20% of the volume of the mixed solvent, and the adding amount of the acid solution is adjusted according to the pH value of the first-stage reaction.
Preferably, the reaction in the first stage is carried out at a stirring speed of 200-600 rpm for 5-8 h; preferably, after the first-stage reaction is finished, adjusting the pH value of the intermediate reaction solution to the target pH value of the second-stage reaction by adding an acid solution or adding water for dilution; more preferably, the second stage reaction is carried out at a stirring speed of 150-500 rpm for 8-12 h. The reaction of two stages is carried out under the process conditions, which is favorable for further improving the reaction stability, and the formed Fe3(PO4)2The loose interior of the precursor crystal is more adaptive to the compact surface layer, and finally the precursor crystal is subjected to LiFePO4After the precursor grows and is roasted, the obtained spherical lithium iron phosphate with the hollow structure has a more complete structure and more proper internal and external pore degrees, and is favorable for further improving the overall electrochemical performance of the anode material.
In the actual operation process, the pH value of the system can be monitored at any time in the first-stage reaction process and the second-stage reaction process, and the pH can be regulated and controlled in the process of supplementing acid solution or dilution water, so that the pH value of the gas during the reaction is ensured to be in a preset range. In the second stageAfter the reaction is finished, the reacted slurry can be centrifuged, dried and purified to obtain Fe3(PO4)2And (3) precursor. The specific impurity removal process is preferably as follows: adding chelate resin into the reaction solution, wherein the addition amount is 0.5-2 g/L, and repeatedly cleaning for 3-4 times by using deionized water and absolute ethyl alcohol.
In the above crystal nucleus growth process, Li3PO4The crystal nucleus can be in Fe3(PO4)2Nucleation and growth are carried out on the surface of the precursor to complete the blending of two structural crystals to form LiFePO4And (3) precursor. In a preferred embodiment, the nucleus growing step includes: adding phosphoric acid solution into lithium hydroxide solution for reaction to form Li3PO4A crystal solution; mixing Fe3(PO4)2Precursor dispersion to Li3PO4Crystal nucleus growth is carried out in the crystal solution to obtain LiFePO4And (3) precursor. By the above-mentioned manner of adding lithium source and Li3PO4Crystal growth mode, formed LiFePO4The precursor has a more complete crystal structure, and is beneficial to further improving the crystal structure of the final spherical lithium iron phosphate, so that the anode material has better electrochemical performance.
Above Fe3(PO4)2Precursor and LiFePO4During the precursor reaction growth process, inert gas such as nitrogen is preferably introduced into the reaction system to remove oxygen in the solution and prevent Fe2+The oxidation of (2) makes the reaction more stable.
Preferably, the phosphoric acid solution is a phosphoric acid aqueous solution, and the molar concentration of the phosphoric acid aqueous solution is 1.0-2.5 mol/L; the lithium hydroxide solution is a lithium hydroxide aqueous solution, and the molar concentration of the lithium hydroxide aqueous solution is 1.0-2.5 mol/L; li in lithium hydroxide solution+With PO in phosphoric acid solution4 3-The molar ratio of (2.85-3.15) to (1); preferably, the adding speed of the phosphoric acid solution is 10-30L/h, the reaction time of the phosphoric acid solution and the lithium hydroxide solution is 2-3 h, and the reaction is carried out at the stirring speed of 150-250 rpm; preferably, Fe3(PO4)2The addition amount of the precursor is Li3PO4By weight of the crystal solution5-10%; preferably, the crystal nucleus growth process is carried out at a stirring speed of 100-150 rpm, and the growth time is 3-5 h. The adoption of the process conditions is beneficial to further improving the LiFePO4Stability of the precursor crystal nucleus growth process.
The above carbon source is only required to be capable of forming a carbon material compounded to the surface of the spherical lithium iron phosphate in an inert atmosphere in the roasting stage, and in a preferred embodiment, the carbon source is selected from one or more of graphene, carbon nanotubes, conductive carbon black and conductive polymers; preferably, the conductive polymer is polyaniline. Polyaniline and other conductive polymers can be pyrolyzed to form amorphous carbon in the roasting process. Preferably, the carbon source is added in an amount of LiFePO45-7% of the total weight of the precursor and the carbon raw material source. With the addition amount, the carbon content in the baked cathode material is more appropriate, and the conductivity of the material can be further improved. Preferably, in the roasting process, the roasting temperature is 600-700 ℃, and the roasting time is 5-7 h. And the spherical lithium iron phosphate is roasted under the conditions, so that the hollow structure of the spherical lithium iron phosphate structure is more complete, and the compounding of the carbon material is more stable.
Before actual roasting, the reaction slurry formed in the crystal nucleus growth step can be centrifuged, dried and purified to obtain LiFePO4And mixing the precursor with a carbon source and roasting. The above inert atmosphere includes, but is not limited to, nitrogen, argon, and the like. The impurity removal process is preferably as follows: adding chelate resin into the reaction solution, wherein the addition amount is 0.5-2 g/L, and repeatedly cleaning for 3-4 times by using deionized water and absolute ethyl alcohol.
According to another aspect of the present invention, a positive electrode material is also provided, which is the lithium iron phosphate positive electrode material, or the lithium iron phosphate positive electrode material prepared by the preparation method.
According to another aspect of the present invention, a lithium ion battery is provided, including a positive electrode, the positive electrode includes a positive electrode current collector and a positive electrode active layer located on the surface of the positive electrode current collector, the positive electrode active layer includes a positive electrode material, a conductive agent and a binder, wherein the positive electrode material is the above-mentioned lithium iron phosphate positive electrode material, or is the lithium iron phosphate positive electrode material prepared by the above-mentioned preparation method.
By adopting the lithium iron phosphate cathode material, the first charge-discharge efficiency, the coulombic efficiency, the rate capability, the long-term cycle performance and the like of the lithium ion battery can be effectively improved, and the lithium iron phosphate cathode material has outstanding capacity, service life, safety and the like.
The present application is described in further detail below with reference to specific examples, which should not be construed as limiting the scope of the invention as claimed.
Example 1
1. Configuring FeCl2And (NH)4)2HPO4The total solute concentration of the binary aqueous solution is 2mol/L, Fe2+And PO4 3+In a molar ratio of 3: 2; the citric acid aqueous solution is taken as a complexing agent solution, and the molar concentration of the citric acid aqueous solution is 3 mol/L; the carbonic acid water solution is used as an acid solution, and the molar concentration of the carbonic acid water solution is 0.2 mol/L;
2. preparing a mixed solvent of ethylene glycol and deionized water as a reaction solution, wherein the volume ratio of the deionized water to the ethylene glycol is 3:1, simultaneously adding the binary aqueous solution, the citric acid aqueous solution and the carbonic acid aqueous solution into the reaction solution, carrying out the reaction of the stage 1 and the stage 2, and continuously blowing high-purity N in the reaction process2To remove oxygen from the solution. Wherein the adding amount of the binary solution is 85% of the volume of the reaction solution, the adding amount of the complexing agent solution is 15% of the volume of the reaction solution, and the adding amount of the acid solution is adjusted according to the following pH value:
stage 1: finely adjusting the addition amount of the carbonic acid solution, regulating and controlling the pH of a reaction system to be 1.8-2.0, controlling the reaction time to be 5h, and controlling the stirring speed to be 500 rpm;
and (2) stage: adding the addition amount of a carbonic acid solution, regulating and controlling the pH value of a reaction system to be 2.8-3.0, controlling the stirring rotating speed to be 350rpm, continuously reacting for 8 hours, and stopping stirring;
3. centrifuging the reacted slurry, drying, and repeatedly cleaning with deionized water and ethanol for 3-4 times to remove impurities to obtain Fe3(PO4)2And (3) precursor.
4. Reacting LiOH and H3PO4Respectively preparing into 1.5mol/L aqueous solution; h is to be3PO4The solution was slowly added to LiOH at a rate of 10L/hIn solution, wherein the molar amount n (Li)+):n(PO4 3-) Is 3:1, and continuously stirring for 2-3 h to obtain Li3PO4And (4) crystals. Mixing the above Fe3(PO4)2The precursor is uniformly dispersed to Li3PO4In a crystal solution of Fe3(PO4)2The addition amount of the precursor is Li3PO4Continuously stirring for 3h with the stirring speed controlled at 150rpm and the adding speed controlled at 10L/h to ensure the growth of crystal nuclei, centrifuging the reacted slurry, drying, and removing impurities to obtain LiFePO4And (3) precursor.
5. Mixing LiFePO4Mixing the precursor and graphene, wherein the addition amount of the graphene is 7% of the total mass of the precursor and the graphene, roasting for 6h at 600 ℃ under the protection of nitrogen in inert atmosphere, and naturally cooling to room temperature to obtain hollow-structure LiFePO4The carbon content of the/C cathode material (XRD structure spectrum is shown in figure 1) is 1.5%.
Example 2
1. Configuration of FeSO4And NH4H2PO4The total solute concentration of the binary aqueous solution is 1.5mol/L, Fe2+And PO4 3+In a molar ratio of 3.15: 2; the oxalic acid aqueous solution is used as a complexing agent solution, and the molar concentration of the oxalic acid aqueous solution is 5 mol/L; the acetic acid water solution is used as an acid solution, and the molar concentration of the acetic acid water solution is 0.2 mol/L;
2. preparing a mixed solvent of polyethylene glycol 200 and deionized water as a reaction solution, wherein the volume ratio of the deionized water to the polyethylene glycol is 2:1, simultaneously adding the binary aqueous solution, the oxalic acid aqueous solution and the acetic acid aqueous solution into the reaction solution, carrying out the reaction of the stage 1 and the stage 2, and continuously blowing high-purity N in the reaction process2To remove oxygen from the solution. Wherein the adding amount of the binary solution is 85% of the volume of the reaction solution, the adding amount of the complexing agent solution is 15% of the volume of the reaction solution, and the adding amount of the acid solution is adjusted according to the following pH value:
stage 1: slightly adjusting the addition amount of the acetic acid solution, regulating and controlling the pH of a reaction system to be 1.9-2.2, controlling the reaction time to be 6h, and controlling the stirring speed to be 400 rpm;
and (2) stage: adding the acetic acid solution, regulating and controlling the pH of the reaction system to be 2.6-2.8, controlling the stirring rotation speed to be 300rpm, continuously reacting for 10 hours, and stopping stirring;
3. centrifuging the reacted slurry, drying, and repeatedly cleaning with deionized water and ethanol for 3-4 times to remove impurities to obtain Fe3(PO4)2And (3) precursor.
4. Reacting LiOH and H3PO4Respectively preparing into 1.5mol/L aqueous solution; h is to be3PO4The solution was slowly added to the LiOH solution at a rate of 20L/h, where the molar amount n (Li)+):n(PO4 3-) Stirring for 2h to obtain Li in a ratio of 3:13PO4And (4) crystals. Mixing the above Fe3(PO4)2The precursor is uniformly dispersed to Li3PO4In a crystal solution of Fe3(PO4)2The addition amount of the precursor is Li3PO4Continuously stirring for 3h with the stirring speed controlled at 100rpm and the adding speed controlled at 10L/h to ensure the growth of crystal nuclei, centrifuging the reacted slurry, drying, and removing impurities to obtain LiFePO4And (3) precursor.
5. Mixing LiFePO4Mixing the precursor and the carbon nano tube, wherein the addition amount of the carbon nano tube is 7 percent of the total mass of the precursor and the carbon nano tube, roasting for 6 hours at 650 ℃ under the protection of nitrogen in inert atmosphere, and naturally cooling to room temperature to obtain hollow-structure LiFePO4The carbon content of the/C cathode material is 2 percent.
Example 3
1. Configuring FeC2O4And Na2HPO4The total solute concentration of the binary aqueous solution is 2mol/L, Fe2+And PO4 3+In a molar ratio of 3: 2; the citric acid aqueous solution is taken as a complexing agent solution, and the molar concentration of the citric acid aqueous solution is 4 mol/L; the phosphoric acid aqueous solution is used as an acid solution, and the molar concentration of the phosphoric acid aqueous solution is 0.2 mol/L;
2. preparing a mixed solvent of glycerol and deionized water as a reaction solution, wherein the volume ratio of the deionized water to the glycerol is 2:1, simultaneously adding the binary aqueous solution, the citric acid aqueous solution and the phosphoric acid aqueous solution into the reaction solution, and carrying out the reaction of the stage 1 and the stage 2, wherein the reaction process is continuously carried outBubbling high purity N2To remove oxygen from the solution. Wherein the adding amount of the binary solution is 90% of the volume of the reaction solution, the adding amount of the complexing agent solution is 10% of the volume of the reaction solution, and the adding amount of the acid solution is adjusted according to the following pH value:
stage 1: slightly adjusting the addition amount of a phosphoric acid solution, regulating and controlling the pH of a reaction system to be 2.0-2.4, controlling the reaction time to be 5h, and controlling the stirring speed to be 450 rpm;
and (2) stage: adding the phosphoric acid solution, regulating and controlling the pH of the reaction system to be 3.0-3.2, controlling the stirring speed to be 300rpm, continuously reacting for 10 hours, and stopping stirring;
3. centrifuging the reacted slurry, drying, and repeatedly cleaning with deionized water and ethanol for 3-4 times to remove impurities to obtain Fe3(PO4)2And (3) precursor.
4. Reacting LiOH and H3PO4Respectively preparing into 1.5mol/L aqueous solution; h is to be3PO4The solution was slowly added to the LiOH solution at a rate of 15L/h, where the molar amount n (Li)+):n(PO4 3-) Stirring for 2h to obtain Li in a ratio of 3:13PO4And (4) crystals. Mixing the above Fe3(PO4)2The precursor is uniformly dispersed to Li3PO4In a crystal solution of Fe3(PO4)2The addition amount of the precursor is Li3PO4Continuously stirring for 3h with the stirring speed controlled at 120rpm and the adding speed controlled at 15L/h to ensure the growth of crystal nuclei, centrifuging the reacted slurry, drying, and removing impurities to obtain LiFePO4And (3) precursor.
5. Mixing LiFePO4Mixing the precursor and conductive carbon black, wherein the addition amount of the conductive carbon black is 7 percent of the total mass of the precursor and the conductive carbon black, roasting for 6 hours at 600 ℃ under the protection of nitrogen in inert atmosphere, and naturally cooling to room temperature to obtain hollow-structure LiFePO4The carbon content of the/C cathode material is 2.2 percent.
Example 4
1. Configuration of FeSO4And NH4H2PO4The total solute concentration of the binary aqueous solution is 2mol/L, Fe2+And PO4 3+In a molar ratio of 2.85: 2; the oxalic acid aqueous solution is used as a complexing agent solution, and the molar concentration of the oxalic acid aqueous solution is 5 mol/L; the phosphoric acid aqueous solution is used as an acid solution, and the molar concentration of the phosphoric acid aqueous solution is 0.4 mol/L;
2. preparing a mixed solvent of ethylene glycol and deionized water as a reaction solution, wherein the volume ratio of the deionized water to the ethylene glycol is 2:1, simultaneously adding the binary aqueous solution, the oxalic acid aqueous solution and the phosphoric acid aqueous solution into the reaction solution, carrying out the reaction of the stage 1 and the stage 2, and continuously blowing high-purity N in the reaction process2To remove oxygen from the solution. Wherein the adding amount of the binary solution is 85% of the volume of the reaction solution, the adding amount of the complexing agent solution is 15% of the volume of the reaction solution, and the adding amount of the acid solution is adjusted according to the following pH value:
stage 1: slightly adjusting the addition amount of a phosphoric acid solution, regulating and controlling the pH of a reaction system to be 2.0-2.2, controlling the reaction time to be 6h, and controlling the stirring speed to be 550 rpm;
and (2) stage: adding the phosphoric acid solution, regulating and controlling the pH of the reaction system to be 3.0-3.2, controlling the stirring rotation speed to be 400rpm, continuously reacting for 9 hours, and stopping stirring;
3. centrifuging the reacted slurry, drying, and repeatedly cleaning with deionized water and ethanol for 3-4 times to remove impurities to obtain Fe3(PO4)2And (3) precursor.
4. Reacting LiOH and H3PO4Respectively preparing into 1.5mol/L aqueous solution; h is to be3PO4The solution was slowly added to the LiOH solution at a rate of 20L/h, where the molar amount n (Li)+):n(PO4 3-) Is 3:1, and continuously stirring for 2-3 h to obtain Li3PO4And (4) crystals. Mixing the above Fe3(PO4)2The precursor is uniformly dispersed to Li3PO4In a crystal solution of Fe3(PO4)2The addition amount of the precursor is Li3PO4Continuously stirring for 3h with the stirring speed controlled at 120rpm and the adding speed controlled at 20L/h to ensure the growth of crystal nuclei, centrifuging the reacted slurry, drying, and removing impurities to obtain LiFePO4And (3) precursor.
5. Mixing LiFePO4Precursor and guideMixing electric carbon black, wherein the addition amount of the conductive carbon black is 7 percent of the total mass of the electric carbon black and the conductive carbon black, roasting for 6 hours at 600 ℃ under the protection of nitrogen in inert atmosphere, and naturally cooling to room temperature to obtain hollow-structure LiFePO4The carbon content of the/C cathode material is 1.8 percent.
Example 5
1. Configuration (CH)3COO)2Fe and NH4H2PO4The total solute concentration of the binary aqueous solution is 2mol/L, Fe2+And PO4 3+In a molar ratio of 2.85: 2; the ascorbic acid water solution is used as a complexing agent solution, and the molar concentration of the ascorbic acid water solution is 4 mol/L; the acetic acid water solution is used as an acid solution, and the molar concentration of the acetic acid water solution is 0.2 mol/L;
2. preparing a mixed solvent of polyethylene glycol 200 and deionized water as a reaction solution, wherein the volume ratio of the deionized water to the polyethylene glycol 200 is 2:1, simultaneously adding the binary aqueous solution, the ascorbic acid aqueous solution and the acetic acid aqueous solution into the reaction solution, carrying out the reaction of the stage 1 and the stage 2, and continuously blowing high-purity N in the reaction process2To remove oxygen from the solution. Wherein the adding amount of the binary solution is 90% of the volume of the reaction solution, the adding amount of the complexing agent solution is 10% of the volume of the reaction solution, and the adding amount of the acid solution is adjusted according to the following pH value:
stage 1: slightly adjusting the addition amount of the acetic acid solution, regulating and controlling the pH of a reaction system to be 1.9-2.2, controlling the reaction time to be 5h, and controlling the stirring speed to be 500 rpm;
and (2) stage: adding the acetic acid solution, regulating and controlling the pH of the reaction system to be 2.9-3.2, controlling the stirring rotation speed to be 300rpm, and stopping stirring after continuously reacting for 9 hours;
3. centrifuging the reacted slurry, drying and removing impurities to obtain Fe3(PO4)2And (3) precursor.
4. Reacting LiOH and H3PO4Respectively preparing into 1.8mol/L aqueous solution; h is to be3PO4The solution was slowly added to the LiOH solution at a rate of 25L/h, where the molar amount n (Li)+):n(PO4 3-) Is 3:1, and continuously stirring for 2-3 h to obtain Li3PO4And (4) crystals. Mixing the above Fe3(PO4)2Precursor bodyIs uniformly dispersed to Li3PO4In a crystal solution of Fe3(PO4)2The addition amount of the precursor is Li3PO4Continuously stirring for 3h with the stirring speed controlled at 150rpm and the adding speed controlled at 25L/h to ensure the growth of crystal nuclei, centrifuging the reacted slurry, drying, and removing impurities to obtain LiFePO4And (3) precursor.
5. Mixing LiFePO4Mixing the precursor and conductive carbon black, wherein the addition amount of the conductive carbon black is 7 percent of the total mass of the precursor and the conductive carbon black, roasting for 6 hours at 600 ℃ under the protection of nitrogen in inert atmosphere, and naturally cooling to room temperature to obtain hollow-structure LiFePO4The carbon content of the/C cathode material is 2 percent.
Example 6
1. Configuring FeC2O4And NH4H2PO4The total solute concentration of the binary aqueous solution is 2mol/L, Fe2+And PO4 3+In a molar ratio of 2.85: 2; the ascorbic acid water solution is used as a complexing agent solution, and the molar concentration of the ascorbic acid water solution is 5 mol/L; the phosphoric acid aqueous solution is used as an acid solution, and the molar concentration of the phosphoric acid aqueous solution is 0.2 mol/L;
2. preparing a mixed solvent of ethylene glycol and deionized water as a reaction solution, wherein the volume ratio of the deionized water to the ethylene glycol is 2:1, simultaneously adding the binary aqueous solution, the ascorbic acid aqueous solution and the phosphoric acid aqueous solution into the reaction solution, carrying out the reaction of the stage 1 and the stage 2, and continuously blowing high-purity N in the reaction process2To remove oxygen from the solution. Wherein the adding amount of the binary solution is 90% of the volume of the reaction solution, the adding amount of the complexing agent solution is 10% of the volume of the reaction solution, and the adding amount of the acid solution is adjusted according to the following pH value:
stage 1: slightly adjusting the addition amount of a phosphoric acid solution, regulating and controlling the pH of a reaction system to be 1.8-2.0, controlling the reaction time to be 6h, and controlling the stirring speed to be 500 rpm;
and (2) stage: adding the phosphoric acid solution, regulating and controlling the pH of the reaction system to be 2.8-3.0, controlling the stirring rotation speed to be 250rpm, continuously reacting for 9 hours, and stopping stirring;
3. centrifuging the reacted slurry, drying and removing impurities to obtain Fe3(PO4)2And (3) precursor.
4. Reacting LiOH and H3PO4Respectively preparing into 2mol/L aqueous solution; h is to be3PO4The solution was slowly added to the LiOH solution at a rate of 18L/h, where the molar amount n (Li)+):n(PO4 3-) Is 3:1, and continuously stirring for 2-3 h to obtain Li3PO4And (4) crystals. Mixing the above Fe3(PO4)2The precursor is uniformly dispersed to Li3PO4In a crystal solution of Fe3(PO4)2The addition amount of the precursor is Li3PO4Continuously stirring for 3h with the stirring speed controlled at 100rpm and the adding speed controlled at 18L/h to ensure the growth of crystal nuclei, centrifuging the reacted slurry, drying, and removing impurities to obtain LiFePO4And (3) precursor.
5. Mixing LiFePO4Mixing the precursor and conductive carbon black, wherein the addition amount of the conductive carbon black is 5 percent of the total mass of the precursor and the conductive carbon black, roasting for 6 hours at 600 ℃ under the protection of nitrogen in inert atmosphere, and naturally cooling to room temperature to obtain hollow-structure LiFePO4The carbon content of the/C cathode material is 2 percent.
Comparative example 1
1. The raw material LiOH, (NH)4)H2PO4，FeC2O4Pouring the mixture into a ball milling tank according to the molar ratio of 1:1:1, and adding a certain amount of absolute ethyl alcohol for uniform dispersion; ball milling is carried out for 8 hours at the speed of 300r/min to obtain suspension with good dispersibility; drying the reacted slurry in a vacuum oven, and removing impurities to obtain LiFePO4And (3) precursor.
2. Mixing LiFePO4Uniformly mixing the precursor with a small amount of conductive carbon black, wherein the mass fraction of the conductive carbon is 7%, roasting for 6h at 600 ℃ under the protection of inert atmosphere, setting the heating rate at 3 ℃/min, and naturally cooling to room temperature to obtain LiFePO4The carbon content of the/C cathode material is 2 percent.
And (3) performance characterization:
and (3) material testing: by using N2Adsorption and desorption specific surface instrument characterization LiFePO4BET value of/C cathode material; by means of a vacuum chamberDensity meter characterization LiFePO4The internal and external porosity of the material is characterized by particle size distribution and internal high porosity part particle size by adopting a laser particle size analyzer, and the thermal stability and safety performance of the material are characterized by adopting a differential scanning calorimeter.
And (3) testing the battery performance: LiFePO prepared in each example and comparative example by using metal lithium as a negative electrode4the/C material is a 2032 button half cell assembled by a positive electrode, a conductive agent (conductive carbon black SP) and a binder (PVDF 5130) according to a formula of 90:5: 5. 1.0mol/L LiPF6And the mixed solution of/EC + EMC + DMC (the volume ratio is EC: EMC: DMC is 1:1) is used as the electrolyte. Aluminum foil is used as a current collector, Celgard 2400 is used as a diaphragm, and a metal lithium sheet is used as a counter electrode. The gram charge-discharge capacity and the first charge-discharge efficiency (%) of the lithium iron phosphate anode material are tested, and the diffusion coefficient (D) of lithium ions is tested by adopting a GITT methodLi +). LiFePO prepared by using commercial graphite as a negative electrode4the/C material is a soft-package battery cell assembled by the positive electrode, and the first charge-discharge efficiency, the rate performance, the cycle performance and the safety performance of the soft-package battery cell are tested; the long cycle voltage range is 2.5-3.65V, the current density is 1℃/1℃, and the test is carried out in a thermostatic chamber at 25 +/-1 ℃.
Fig. 1 shows an XRD structure diagram corresponding to the lithium iron phosphate cathode material prepared in example 1;
fig. 2 shows a first charge-discharge curve (0.1C, 2.0-3.7V) of the button cell corresponding to the lithium iron phosphate positive electrode material prepared in example 1;
fig. 3 shows a rate performance curve of a soft-package cell corresponding to the lithium iron phosphate positive electrode material prepared in example 1;
FIG. 4 shows the GITT curve of button cell corresponding to the lithium iron phosphate positive electrode material prepared in example 1, and the ordinate of the GITT curve is the diffusion coefficient value of lithium ion, and the diffusion coefficient value is expressed in (cm)2S); the abscissa is the lithium removal depth of the lithium iron phosphate and is represented by SOC%;
fig. 5 shows a 45 ℃ circulation capacity retention rate curve (1C/1C 2.5-3.65V) of a soft-package battery cell corresponding to the lithium iron phosphate positive electrode material prepared in example 1;
fig. 6 shows a DSC thermal analysis graph corresponding to the lithium iron phosphate positive electrode material prepared in example 1;
fig. 7 shows a first charge-discharge curve (0.1C, 2.0-3.7V) of the button cell corresponding to the lithium iron phosphate positive electrode material prepared in comparative example 1;
the characterization results are shown in Table 1:
TABLE 1
Compared with the first charge-discharge curves in fig. 2 and fig. 7, the charge-discharge capacity value of the lithium iron phosphate anode material in fig. 2 is higher than that in fig. 7, and the first charge-discharge efficiency is higher. In addition, the voltage difference between the charging and discharging voltage platforms in fig. 2 is small, and the voltage difference between the charging and discharging voltage platforms in fig. 7 is large, so that an obvious electrode polarization phenomenon exists, and the subsequent cycle performance is influenced by the large polarization. The above tests show that the electrochemical performance of the lithium iron phosphate cathode material in example 1 is superior to that of the cathode material in comparative example 1.
As can also be seen from the data in table 1, the lithium iron phosphate prepared in the above examples 1 to 6 has a distinct hollow structure, and the porosity of the surface layer is much lower than that of the core. The surface of the carbon-doped lithium ion battery is compounded with carbon and then used as a lithium battery anode material, so that the charge-discharge efficiency, the capacity retention rate and the cycle performance of the battery can be obviously improved, and the battery is correspondingly promoted to have longer service life by better cycle performance. Meanwhile, the lithium iron phosphate has better thermal stability, and is applied to the post-battery solution, thereby being beneficial to improving the safety of the battery.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. The utility model provides a lithium iron phosphate cathode material, its characterized in that, lithium iron phosphate cathode material includes spherical lithium iron phosphate and cladding and is in the carbon on spherical lithium iron phosphate surface, spherical lithium iron phosphate has the kernel and is located kernel outlying top layer, the carbon cladding is in on the top layer, the porosity of kernel is greater than the porosity on top layer.
2. The lithium iron phosphate positive electrode material according to claim 1, wherein the spherical lithium iron phosphate has a particle size of 4 to 6 μm; the particle size of the inner core is 1-3 mu m.
3. The lithium iron phosphate positive electrode material according to claim 1 or 2, wherein the porosity of the core is 60 to 90%; the surface layer has a porosity of 20% or less.
4. The lithium iron phosphate positive electrode material according to claim 1 or 2, wherein the carbon is selected from one or more of graphene, carbon nanotubes, conductive carbon black, amorphous carbon; the content of the carbon is 1.5-2.5% of the weight of the lithium iron phosphate anode material.
5. A preparation method of a lithium iron phosphate positive electrode material is characterized by comprising the following steps:
preparing a mixed solvent of water and an organic solvent, adding a binary solution, a complexing agent solution and an acid solution into the mixed solvent, and carrying out a first-stage reaction under the condition that the pH value is 1.8-2.4 to form an intermediate reaction solution; adjusting the pH value of the intermediate reaction solution to 2.6-3.2, and carrying out a second stage reaction to form Fe3(PO4)2A precursor; wherein the binary solution is a solution of soluble ferrous salt and a soluble compound containing phosphate ions;
subjecting said Fe to3(PO4)2Precursor dispersion to Li3PO4Crystal nucleus growth is carried out in the crystal solution to obtain LiFePO4A precursor;
reacting said LiFePO4And mixing the precursor with a carbon source, and then roasting in an inert atmosphere to obtain the lithium iron phosphate anode material.
6. The method according to claim 5, wherein the organic solvent in the mixed solvent is an alcohol solvent, and the volume ratio of water to the organic solvent in the mixed solvent is (1-3): 1-3.
7. The method according to claim 5 or 6, wherein the soluble ferrous salt is selected from FeCl2、FeC2O4、(CH3COO)2Fe、FeSO4One or more of;
the soluble compound containing phosphate ions is selected from phosphoric acid and/or soluble phosphate, and the soluble phosphate is selected from NH4H2PO4，Na2HPO4，(NH4)2HPO4One or more of;
the binary solution is the water solution of the soluble ferrous salt and the soluble compound containing phosphate radical ions, wherein Fe in the soluble ferrous salt2+With PO of said phosphate ion-containing soluble compound4 3+The molar ratio of (2.85-3.15) to (2), and the total molar concentration of the soluble ferrous salt and the phosphate radical-containing ions is 1-3 mol/L;
in the complexing agent solution, the complexing agent is selected from one or more of citric acid, oxalic acid and ascorbic acid.
8. The production method according to claim 5 or 6, wherein the crystal nucleus growing step includes:
adding phosphoric acid solution into lithium hydroxide solution for reaction to form the Li3PO4A crystal solution;
subjecting said Fe to3(PO4)2Precursor is dispersed to the Li3PO4Crystal nucleus growth is carried out in the crystal solution to obtain the LiFePO4A precursor;
the phosphoric acid solution is a phosphoric acid water solution, and the molar concentration of the phosphoric acid solution is 1.0-2.5 mol/L; the lithium hydroxide solution is a lithium hydroxide aqueous solution, and the molar concentration of the lithium hydroxide aqueous solution is 1.0-2.5 mol/L; the lithium hydroxideLi in solution+With PO in the phosphoric acid solution4 3-The molar ratio of (2.85-3.15): 1.
9. The lithium iron phosphate positive electrode material is characterized by being the lithium iron phosphate positive electrode material in any one of claims 1 to 4 or the lithium iron phosphate positive electrode material prepared by the preparation method in any one of claims 5 to 8.
10. A lithium ion battery comprises a positive electrode, wherein the positive electrode comprises a positive electrode current collector and a positive electrode active layer positioned on the surface of the positive electrode current collector, and the positive electrode active layer comprises a positive electrode material, a conductive agent and a binder, and is characterized in that the positive electrode material is the lithium iron phosphate positive electrode material in any one of claims 1 to 4 or the lithium iron phosphate positive electrode material prepared by the preparation method in any one of claims 5 to 8.
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