CN113097455B - Modified lithium iron phosphate composite material, positive electrode material and preparation method thereof - Google Patents

Modified lithium iron phosphate composite material, positive electrode material and preparation method thereof Download PDF

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CN113097455B
CN113097455B CN202110204194.7A CN202110204194A CN113097455B CN 113097455 B CN113097455 B CN 113097455B CN 202110204194 A CN202110204194 A CN 202110204194A CN 113097455 B CN113097455 B CN 113097455B
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lithium
iron phosphate
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lithium iron
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CN113097455A (en
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王瑛
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Anhui Jietu New Energy Technology Co ltd
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Yunnan Hangkai Technology Co ltd
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    • 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
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Abstract

The invention relates to the technical field of lithium ion battery cathode materials, in particular to a modified lithium iron phosphate composite material, a cathode material and a preparation method thereof. The modified lithium iron phosphate composite material comprises a core and a coating layer coated on the outer surface of the core, wherein the core comprises carbon and lithium iron phosphate, and the coating layer comprises carbon and lithium oxalate; according to the mode, the lithium iron phosphate and the lithium oxalate form a nano core-shell structure, and the lithium oxalate is coated outside the lithium iron phosphate, so that the lithium oxalate can be decomposed at a lower potential without adding other catalysts, and meanwhile, carbon is contained in the inner core and the coating layer, so that the decomposition kinetic property and the conductivity of the lithium oxalate are improved, and the complete decomposition of the lithium oxalate is facilitated; the modified lithium iron phosphate composite material is coated on the positive electrode, and is beneficial to improving the capacity of the lithium ion battery.

Description

Modified lithium iron phosphate composite material, positive electrode material and preparation method thereof
Technical Field
The invention relates to the technical field of lithium ion battery cathode materials, in particular to a modified lithium iron phosphate composite material, a cathode material and a preparation method thereof.
Background
The current lithium ion battery systems are all assembled in a discharge state, and lithium ions are all from the positive electrode. For graphite negative electrode materials, due to the formation of an SEI film during the first charging, about 10% of lithium ions are consumed, and about 10% of capacity loss is caused, so that the increase of the capacity of the lithium ion battery is greatly limited. The lithium supplement technology is to additionally provide a part of lithium to offset lithium consumed by the SEI film, so as to improve the capacity of the lithium ion battery. The lithium supplementing technology comprises anode lithium supplementing and cathode lithium supplementing, wherein the anode lithium supplementing mainly comprises a material with high charge and low discharge or a compound which decomposes and releases lithium and gas lithium at high potential, and common lithium compounds for supplementing lithium comprise lithium azide, squaric acid lithium salt, lithium oxalate and the like.
In the prior art, when lithium oxalate is used as a positive electrode lithium supplement material, the lithium oxalate needs to be mixed with a catalyst and then coated on the surface of the positive electrode material when being applied to a lithium iron phosphate system due to the high decomposition potential of the lithium oxalate.
Disclosure of Invention
The invention aims to provide a modified lithium iron phosphate composite material, a positive electrode material and a preparation method thereof, and aims to solve the technical problem that in the prior art, lithium oxalate serving as a lithium supplement material has high decomposition potential and needs to be added with a catalyst or a conductive agent.
The technical scheme of the invention is as follows: the modified lithium iron phosphate composite material comprises a core and a coating layer coated on the outer surface of the core, wherein the core comprises carbon and lithium iron phosphate, and the coating layer comprises carbon and lithium oxalate; the mass ratio of the carbon in the core, the carbon in the coating layer, the lithium oxalate in the coating layer and the core is (1.95-3): (0.5-1.0): (1.0-4): (92-100).
Preferably, the core is carbon-coated doped lithium iron phosphate, the lithium iron phosphate in the core is doped lithium iron phosphate, and the general formula of the core is LiFeγM1-γPO4and/C, wherein M is one or more of vanadium V, niobium Nb, titanium Ti, tungsten W, lanthanum La, yttrium Y, zirconium Zr or magnesium Mg, and gamma is more than or equal to 0.8 and less than or equal to 1.
Preferably, the core is carbon-coated doped lithium iron phosphate, and the lithium iron phosphate in the core is dopedThe general formula of the inner core is LiFeαM1-αPO4and/C, wherein M is manganese Mn, and alpha is more than or equal to 0.2 and less than or equal to 1.
The other technical scheme of the invention is as follows: provided is a preparation method of a modified lithium iron phosphate composite material, comprising the following steps:
s1, adding a lithium source, an iron source, a phosphorus source and a first carbon source into the first solvent, and mixing to obtain a first mixture;
s2, grinding the first mixture by using a grinder to obtain first slurry;
s3, spray drying the first slurry to obtain first powder;
s4, roasting the first powder in an inert gas atmosphere to obtain carbon-coated lithium iron phosphate;
s5, adding lithium oxalate, the carbon-coated lithium iron phosphate and a second carbon source into a second solvent for mixing to obtain a second mixture;
s6, spray drying the second mixture to obtain second powder;
s7, roasting the second powder in an inert gas atmosphere to obtain a modified lithium iron phosphate composite material;
wherein the mass ratio of the residual amount of the carbon element in the first carbon source after roasting to the carbon-coated lithium iron phosphate is (1.95-3): (92-100); the ratio of the mass of the lithium oxalate to the mass of the carbon-coated lithium iron phosphate is (1.0-4): (92-100); the mass ratio of the residual amount of the carbon element in the second carbon source after roasting to the carbon-coated lithium iron phosphate is (0.5-1.0): (92-100).
Preferably, the carbon-coated lithium iron phosphate is carbon-coated doped lithium iron phosphate;
step S1 specifically includes: adding a lithium source, an iron source, a doped metal source, a phosphorus source and a first carbon source into a first solvent, and mixing to obtain a first mixture, wherein the molar ratio of the elements of the lithium source, the iron source, the doped metal source and the phosphorus source is 1: γ: (1-. gamma.): 1, wherein gamma is more than or equal to 0.8 and less than or equal to 1, and the doped metal is one or more of vanadium V, niobium Nb, titanium Ti, tungsten W, lanthanum La, yttrium Y, zirconium Zr or magnesium Mg.
Preferably, the carbon-coated lithium iron phosphate is carbon-coated doped lithium iron phosphate;
step S1 specifically includes: adding a lithium source, an iron source, a doped metal source, a phosphorus source and a first carbon source into a first solvent, and mixing to obtain a first mixture, wherein the molar ratio of the elements of the lithium source, the iron source, the doped metal source and the phosphorus source is 1: γ: (1-. gamma.): 1, wherein gamma is more than or equal to 0.2 and less than or equal to 1, and M is manganese Mn.
Preferably, the roasting temperature of the first powder is 650-795 ℃; the roasting temperature of the second powder is 350-450 ℃; the inert gas is selected from one or more of argon, helium, nitrogen and carbon dioxide;
and/or the grinding particle size D50 of the first slurry is 0.25-0.5 μm.
Preferably, the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium citrate and lithium dihydrogen phosphate; the iron source is selected from one or more of ferric oxide, ferroferric oxide, ferric citrate, ferric phosphate and ferrous oxalate;
and/or the phosphorus source is selected from one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid and lithium dihydrogen phosphate;
and/or the first carbon source or the second carbon source is selected from one or more of glucose, sucrose, citric acid, polyethylene glycol, polyvinyl butyral, polyvinyl alcohol, polyacrylonitrile, starch, cellulose;
and/or the first solvent is an organic solvent or water, the organic solvent is selected from one of methanol, ethanol, acetone and N-methyl-2-pyrrolidone, and the second solvent is water;
and/or the doping metal source is magnesium oxalate or mangano-manganic oxide or titanium oxide.
The other technical scheme of the invention is as follows: the modified lithium iron phosphate cathode material is characterized by comprising a modified lithium iron phosphate composite material, a conductive agent, PVDF and NMP, wherein the modified lithium iron phosphate composite material is prepared from the modified lithium iron phosphate composite material or the modified lithium iron phosphate composite material according to the preparation method.
The other technical scheme of the invention is as follows: the preparation method of the modified lithium iron phosphate cathode material comprises the following steps:
stirring and mixing the modified lithium iron phosphate composite material, a conductive agent, PVDF and NMP to obtain anode slurry;
coating the positive electrode slurry on an aluminum foil to obtain a positive electrode material;
drying the positive electrode material under vacuum, pressing the dried positive electrode material into a sheet shape, wherein the thickness of the sheet positive electrode material is less than 0.3 mm;
the modified lithium iron phosphate composite material is prepared by the preparation method.
The invention has the beneficial effects that: the modified lithium iron phosphate composite material comprises a core and a coating layer coated on the outer surface of the core, wherein the core comprises carbon and lithium iron phosphate, and the coating layer comprises carbon and lithium oxalate; according to the mode, the lithium iron phosphate and the lithium oxalate form a nano core-shell structure, and the lithium oxalate is coated outside the lithium iron phosphate, so that the lithium oxalate can be decomposed at a lower potential without adding other catalysts, and meanwhile, carbon is contained in the inner core and the coating layer, so that the decomposition kinetic property and the conductivity of the lithium oxalate are improved, and the complete decomposition of the lithium oxalate is facilitated; the modified lithium iron phosphate composite material is coated on the positive electrode, and is beneficial to improving the capacity of the lithium ion battery.
[ description of the drawings ]
Fig. 1 is a DSEM diagram of a modified lithium iron phosphate composite of example 2 of the present invention.
[ detailed description ] embodiments
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings and specific 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.
In order to make the description of the present disclosure more complete and complete, the following description is given for illustrative purposes with respect to the embodiments and examples of the present invention; it is not intended to be the only form in which the embodiments of the invention may be practiced or utilized. The embodiments are intended to cover the features of the various embodiments as well as the method steps and sequences for constructing and operating the embodiments. However, other embodiments may be utilized to achieve the same or equivalent functions and step sequences.
The term "average particle size D50" as used herein means the equivalent diameter of the largest particle in the particle size distribution curve at 50% cumulative distribution, and has the physical meaning that 50% are particles having a particle size of less than D50, 50% are particles having a particle size of greater than D50, and D50 is also referred to as the median particle size.
In the specification, the description of "a 1 to a 2" means "greater than or equal to a 1" and "less than or equal to a 2", for example, x is 0 to 1, that is: x is more than or equal to 0 and less than or equal to 1.
In order to facilitate understanding of the present invention, in the examples of the present invention, a description will be given of a preparation method and then a description will be given of a product.
Composite Material and examples of preparation of composite Material
The embodiment of the invention provides a preparation method of a modified lithium iron phosphate composite material, which comprises the following steps:
s101, adding a lithium source, an iron source, a phosphorus source and a first carbon source into a first solvent for mixing to obtain a first mixture;
wherein the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium citrate and lithium dihydrogen phosphate; the iron source is selected from one or more of ferric oxide, ferroferric oxide, ferric citrate, ferric phosphate and ferrous oxalate; the phosphorus source is selected from one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid and lithium dihydrogen phosphate.
Wherein the first carbon source is selected from one or more of glucose, sucrose, citric acid, polyethylene glycol, polyvinyl butyral, polyvinyl alcohol, polyacrylonitrile, starch and cellulose.
The first solvent is an organic solvent or water, and the organic solvent is selected from one of methanol, ethanol, acetone and N-methyl-2-pyrrolidone.
S102, grinding the first mixture by using a grinder to obtain first slurry;
wherein the first slurry has a grinding particle diameter D50 of 0.25-0.5 μm.
S103, carrying out spray drying on the first slurry to obtain first powder;
s104, roasting the first powder in an inert gas atmosphere to obtain carbon-coated lithium iron phosphate;
the roasting temperature of the first powder is 650-795 ℃, inert gas is used as protective gas, and the first powder is roasted under the protective atmosphere, wherein the inert gas is one or more of argon, helium, nitrogen and carbon dioxide. And S104, obtaining the carbon-coated lithium iron phosphate as the inner core of the modified lithium iron phosphate composite material.
S105, adding lithium oxalate, the carbon-coated lithium iron phosphate and a second carbon source into a second solvent for mixing to obtain a second mixture;
wherein the second solvent is water, and the second carbon source is selected from one or more of glucose, sucrose, citric acid, polyethylene glycol, polyvinyl butyral, polyvinyl alcohol, polyacrylonitrile, starch and cellulose. Specifically, in step S105, first, lithium oxalate is dissolved in a second solvent to form a lithium oxalate solution; and then adding the carbon-coated lithium iron phosphate and a second carbon source into the lithium oxalate solution, and uniformly stirring and scattering to obtain a second mixture.
S106, carrying out spray drying on the second mixture to obtain second powder;
s107, roasting the second powder in an inert gas atmosphere to obtain a modified lithium iron phosphate composite material;
and roasting the second powder in a protective atmosphere by taking inert gas as protective gas, wherein the roasting temperature of the second powder is 350-450 ℃, and the inert gas is selected from one or more of argon, helium, nitrogen and carbon dioxide.
In steps S105 to S107, lithium oxalate and carbon are coated on the outer surface of the carbon-coated lithium iron phosphate obtained in step S104, and the lithium oxalate and the carbon form a coating layer.
In this embodiment, in the modified lithium iron phosphate composite material obtained in step S107, the mass ratio of the residual amount of carbon elements in the first carbon source after baking to the carbon-coated lithium iron phosphate is (1.95-3): (92-100); the ratio of the mass of the lithium oxalate to the mass of the carbon-coated lithium iron phosphate is (1.0-4): (92-100); the mass ratio of the residual amount of carbon elements after roasting in the second carbon source to the carbon-coated lithium iron phosphate is (0.5-1.0): (92-100).
In this embodiment, the decomposition process of the first carbon source and the second carbon source during the calcination process is complex, the carbon residue after the calcination of different carbon sources and the carbon content in the carbon source are greatly different, and then an additional carbon source is required for the trivalent iron source to perform the reduction. Therefore, in this example, the first carbon source or the second carbon source is added in an amount to ensure that the carbon content in the product after calcination reaches the above-mentioned mass ratio. Specifically, for a specific carbon source, such as glucose, the loss amount of carbon element in the carbon source after calcination can be determined according to an empirical value, and the specific carbon source addition amount can be calculated according to the amount of residual carbon in the product after calcination and the empirical value of the loss amount, so as to back-predict the addition amount of the first carbon source or the second carbon source. Further, when the iron source is ferric iron, the amount of the carbon source required for reducing the ferric iron, the amount of residual carbon after roasting in the product and the empirical value of the loss amount are calculated according to the content of the ferric iron in the iron source, and the addition amount of the first carbon source is reversely deduced.
In an optional embodiment, the carbon-coated lithium iron phosphate is carbon-coated doped lithium iron phosphate. Correspondingly, step S1 specifically includes: adding a lithium source, an iron source, a doped metal source, a phosphorus source and a first carbon source into a first solvent, and mixing to obtain a first mixture, wherein the molar ratio of the elements of the lithium source, the iron source, the doped metal source and the phosphorus source is 1: γ: (1-. gamma.): 1, wherein gamma is more than or equal to 0.8 and less than or equal to 1, and the doped metal is one or more of vanadium V, niobium Nb, titanium Ti, tungsten W, lanthanum La, yttrium Y, zirconium Zr or magnesium Mg. Alternatively, step S1 specifically includes: adding a lithium source, an iron source, a doped metal source, a phosphorus source and a first carbon source into a first solvent, and mixing to obtain a first mixture, wherein the molar ratio of the elements of the lithium source, the iron source, the doped metal source and the phosphorus source is 1: α: (1-. alpha.): 1, wherein alpha is more than or equal to 0.2 and less than or equal to 1, and M is manganese Mn.
The modified lithium iron phosphate composite material prepared according to the preparation method comprises a core and a coating layer coated on the outer surface of the core, wherein the core comprises carbon and lithium iron phosphate, and the coating layer comprises carbon and lithium oxalate; the mass ratio of the carbon in the core, the carbon in the coating layer, the lithium oxalate in the coating layer and the core is (1.95-3): (0.5-1.0): (1.0-4): (92-100).
In this embodiment, the carbon in the core is the residue of the carbon element in the first carbon source after being baked, and the carbon in the cladding layer is the residue of the carbon element in the second carbon source after being baked.
In an optional embodiment, the core is carbon-coated doped lithium iron phosphate, the lithium iron phosphate in the core is doped lithium iron phosphate, and the general formula of the core is LiFeαM1-αPO4and/C, wherein M is manganese Mn, and alpha is more than or equal to 0.2 and less than or equal to 1.
In another optional embodiment, the core is carbon-coated doped lithium iron phosphate, the lithium iron phosphate in the core is doped lithium iron phosphate, and the general formula of the core is LiFeγM1-γPO4and/C, wherein M is one or more of vanadium V, niobium Nb, titanium Ti, tungsten W, lanthanum La, yttrium Y, zirconium Zr or magnesium Mg, and gamma is more than or equal to 0.8 and less than or equal to 1. The modified lithium iron phosphate composite material of the embodiment comprises a core and a coating layer coated on the outer surface of the core, wherein the core comprises carbon and lithium iron phosphate, and the coating layer comprises carbonAnd lithium oxalate; through the mode, lithium iron phosphate and lithium oxalate form a nano core-shell structure, and lithium oxalate is coated outside the lithium iron phosphate, so that the lithium oxalate can be decomposed at a lower potential without adding other catalysts, and meanwhile, carbon is contained in the inner core and the coating layer, so that the decomposition dynamic performance and the conductivity of the lithium oxalate are improved, and the complete decomposition of the lithium oxalate is facilitated.
Examples of preparation of cathode Material and cathode Material
The embodiment of the invention also provides a preparation method of the modified lithium iron phosphate cathode material, which comprises the following steps:
s201, stirring and mixing the modified lithium iron phosphate composite material, a conductive agent, polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) to obtain positive electrode slurry;
the modified lithium iron phosphate composite material is the modified lithium iron phosphate composite material or the composite material prepared by the preparation method.
Specifically, the positive electrode slurry included 100 parts by weight of the modified lithium iron phosphate composite, 1 part by weight of the conductive agent, 2.5 parts by weight of PVDF, and 100 parts by weight of NMP; the conductive agent includes Carbon Nanotubes (CNTs) and graphene in a mass ratio of 1: 1.
S202, coating the anode slurry on an aluminum foil to obtain an anode material;
wherein, the aluminum foil is an aluminum foil with the diameter of 12mm, and the anode slurry is coated on one surface of the aluminum foil.
S203, drying the positive electrode material in vacuum, pressing the dried positive electrode material into a sheet shape, wherein the thickness of the sheet positive electrode material is less than 0.3 mm;
specifically, a sheet-shaped positive electrode material, a metal lithium sheet, a diaphragm and an electrolyte are assembled into a button cell for charge and discharge experiments.
The modified lithium iron phosphate cathode material prepared by the preparation method comprises a modified lithium iron phosphate composite material, PVDF and NMP.
In an alternative embodiment, the cathode material further includes a conductive agent, and the cathode material includes 100 parts by weight of the modified lithium iron phosphate composite, 1 part by weight of the conductive agent, 2.5 parts by weight of PVDF, and 100 parts by weight of NMP; the conductive agent comprises carbon nanotubes and graphene in a mass ratio of 1: 1.
Example 1
The modified lithium iron phosphate composite material provided by the embodiment is prepared according to the following steps: firstly, adding water in a proper proportion into anhydrous iron phosphate, lithium carbonate and glucose according to a mass ratio of 1:0.25:0.14, and uniformly stirring and dispersing; secondly, grinding by using a grinder, and controlling the grinding particle size D50 of the slurry to be 0.45 mu m; thirdly, spray drying is carried out; fourthly, roasting the dried powder for 8 hours at 700 ℃ under the nitrogen protection atmosphere to obtain a lithium iron phosphate core; fifthly, completely dissolving the lithium oxalate with water, then adding the lithium iron phosphate core and a second carbon source, and uniformly stirring and dispersing. Wherein the weight ratio of the lithium oxalate to the lithium iron phosphate sample is 1.0: 100, wherein the adding amount of the second carbon source is 0.5-1.0% of the mass ratio of the carbon content in the coating layer to the lithium iron phosphate core after roasting; and then spray drying, roasting at the roasting temperature of 350-450 ℃ in a protective atmosphere to obtain lithium oxalate and carbon-coated lithium iron phosphate.
Then, preparing a modified lithium iron phosphate positive electrode material by using the modified lithium iron phosphate composite material, and preparing the modified lithium iron phosphate positive electrode material according to the following steps:
sixthly, mixing the following components in percentage by mass of 100: 1: 2.5: 100, uniformly mixing the modified lithium iron phosphate composite material with a conductive agent (CNT: graphene =5: 5), PVDF and NMP to obtain a positive electrode slurry, coating the positive electrode slurry on one surface of an aluminum foil, drying the aluminum foil in vacuum at 110 ℃, pressing the aluminum foil into a wafer with the thickness of less than 0.3mm and the diameter of 12mm to serve as a positive electrode, and then assembling the wafer, a metal lithium sheet, a diaphragm and electrolyte into a button cell to perform charge and discharge tests.
Example 2
The modified lithium iron phosphate composite material provided by the embodiment is prepared according to the following steps: firstly, adding water in a proper proportion into anhydrous iron phosphate, lithium carbonate and glucose according to a mass ratio of 1:0.25:0.14, and uniformly stirring and dispersing; secondly, grinding by using a grinder, and controlling the grinding particle size D50 of the slurry to be 0.45 mu m; thirdly, spray drying is carried out; fourthly, roasting the dried powder for 8 hours at 700 ℃ under the nitrogen protection atmosphere to obtain a lithium iron phosphate core; fifthly, completely dissolving the lithium oxalate with water, then adding the lithium iron phosphate core and a second carbon source, and uniformly stirring and dispersing. Wherein the weight ratio of the lithium oxalate to the lithium iron phosphate sample is 4.0: 100, wherein the adding amount of the second carbon source is 0.5-1.0% of the mass ratio of the carbon content in the coating layer to the lithium iron phosphate core after roasting; and then spray drying, roasting at the roasting temperature of 350-450 ℃ in a protective atmosphere to obtain lithium oxalate and carbon-coated lithium iron phosphate. The SEM image of the resulting modified lithium iron phosphate composite is shown in fig. 1.
Then, preparing a modified lithium iron phosphate positive electrode material by using the modified lithium iron phosphate composite material, and preparing the modified lithium iron phosphate positive electrode material according to the following steps:
sixthly, mixing the following components in percentage by mass of 100: 1: 2.5: 100, uniformly mixing the modified lithium iron phosphate composite material with a conductive agent (CNT: graphene =5: 5), PVDF and NMP to obtain a positive electrode slurry, coating the positive electrode slurry on one surface of an aluminum foil, drying the aluminum foil in vacuum at 110 ℃, pressing the aluminum foil into a wafer with the thickness of less than 0.3mm and the diameter of 12mm to serve as a positive electrode, and then assembling the wafer, a metal lithium sheet, a diaphragm and electrolyte into a button cell to perform charge and discharge tests.
Example 3
The modified lithium iron phosphate composite material provided by the embodiment is prepared according to the following steps: firstly, anhydrous iron phosphate, ammonium metavanadate, lithium carbonate and glucose are mixed according to a mass ratio of 1: 0.01: the ratio of 0.25 to 0.14 is matched with water with proper ratio to be stirred and dispersed evenly; secondly, grinding by using a grinder, and controlling the grinding particle size D50 of the slurry to be 0.45 mu m; thirdly, spray drying is carried out; fourthly, roasting the dried powder for 8 hours at 700 ℃ under the nitrogen protection atmosphere to obtain a lithium iron phosphate core; fifthly, completely dissolving the lithium oxalate with water, then adding the lithium iron phosphate core and a second carbon source, and uniformly stirring and dispersing. Wherein the weight ratio of the lithium oxalate to the lithium iron phosphate sample is 1.0: 100, wherein the adding amount of the second carbon source is 0.5-1.0% of the mass ratio of the carbon content in the coating layer to the lithium iron phosphate core after roasting; and then spray drying, roasting at the roasting temperature of 350-450 ℃ in a protective atmosphere to obtain lithium oxalate and carbon-coated lithium iron phosphate.
Then, preparing a modified lithium iron phosphate positive electrode material by using the modified lithium iron phosphate composite material, and preparing the modified lithium iron phosphate positive electrode material according to the following steps:
sixthly, mixing the following components in percentage by mass of 100: 1: 2.5: 100, uniformly mixing the modified lithium iron phosphate composite material with a conductive agent (CNT: graphene =5: 5), PVDF and NMP to obtain a positive electrode slurry, coating the positive electrode slurry on one surface of an aluminum foil, drying the aluminum foil in vacuum at 110 ℃, pressing the aluminum foil into a wafer with the thickness of less than 0.3mm and the diameter of 12mm to serve as a positive electrode, and then assembling the wafer, a metal lithium sheet, a diaphragm and electrolyte into a button cell to perform charge and discharge tests.
Example 4
The modified lithium iron phosphate composite material provided by the embodiment is prepared according to the following steps: firstly, anhydrous iron phosphate, titanium oxide, lithium carbonate and glucose are mixed according to a mass ratio of 1: 0.01: the ratio of 0.25 to 0.14 is matched with water with proper ratio to be stirred and dispersed evenly; secondly, grinding by using a grinder, and controlling the grinding particle size D50 of the slurry to be 0.45 mu m; thirdly, spray drying is carried out; fourthly, roasting the dried powder for 8 hours at 700 ℃ under the nitrogen protection atmosphere to obtain a lithium iron phosphate core; fifthly, completely dissolving the lithium oxalate with water, then adding the lithium iron phosphate core and a second carbon source, and uniformly stirring and dispersing. Wherein the weight ratio of the lithium oxalate to the lithium iron phosphate sample is 4.0: 100, wherein the adding amount of the second carbon source is 0.5-1.0% of the mass ratio of the carbon content in the coating layer to the lithium iron phosphate core after roasting; and then spray drying, roasting at the roasting temperature of 350-450 ℃ in a protective atmosphere to obtain lithium oxalate and carbon-coated lithium iron phosphate.
Then, preparing a modified lithium iron phosphate positive electrode material by using the modified lithium iron phosphate composite material, and preparing the modified lithium iron phosphate positive electrode material according to the following steps:
sixthly, mixing the following components in percentage by mass of 100: 1: 2.5: 100, uniformly mixing the modified lithium iron phosphate composite material with a conductive agent (CNT: graphene =5: 5), PVDF and NMP to obtain a positive electrode slurry, coating the positive electrode slurry on one surface of an aluminum foil, drying the aluminum foil in vacuum at 110 ℃, pressing the aluminum foil into a wafer with the thickness of less than 0.3mm and the diameter of 12mm to serve as a positive electrode, and then assembling the wafer, a metal lithium sheet, a diaphragm and electrolyte into a button cell to perform charge and discharge tests.
Comparative example 1
Prior art 1: and directly adding lithium oxalate serving as a lithium supplement compound in the battery mixing process, coating, and tabletting to obtain the positive plate containing Li2C2O 4.
Is characterized in that: lithium oxalate is directly mixed with lithium iron phosphate, a conductive agent and a binder as a lithium supplement compound.
The manufacturing process comprises the following steps: firstly, adding water in a proper proportion into anhydrous iron phosphate, lithium carbonate and glucose according to a mass ratio of 1:0.25:0.14, and uniformly stirring and dispersing; secondly, grinding by using a grinder, and controlling the grinding particle size D50 of the slurry to be 0.45 um; thirdly, spray drying is carried out; fourthly, roasting the dried powder for 8 hours at 700 ℃ under the nitrogen protection atmosphere to obtain a lithium iron phosphate sample; ② according to the mass ratio of 100: 1: 2.5: and 4, respectively stirring and uniformly mixing the lithium iron phosphate positive active material with a conductive agent (CNT: graphene =5: 5), PVDF, lithium oxalate and a certain amount of NMP to obtain positive slurry, coating the positive slurry on one surface of an Al foil, drying the Al foil in vacuum at 110 ℃, pressing the Al foil into a wafer with the thickness of less than 0.3mm and phi 12mm to serve as a positive electrode, and assembling the wafer, a lithium metal sheet, a diaphragm and electrolyte into the button cell to perform charge and discharge tests.
Comparative example 2
In the prior art 2, lithium oxalate lithium supplement slurry is prepared, then the lithium oxalate lithium supplement slurry is coated on a lithium iron phosphate positive plate, and the lithium iron phosphate positive plate is dried in a vacuum oven and then used as a positive plate to assemble a button cell.
Is characterized in that: the positive plate is coated twice and has two layers. The bottom layer is a lithium iron phosphate anode mixture, and the upper layer is a lithium oxalate lithium supplement mixture.
The manufacturing process comprises the following steps: firstly, adding water in a proper proportion into anhydrous iron phosphate, lithium carbonate and glucose according to a mass ratio of 1:0.25:0.14, and uniformly stirring and dispersing; secondly, grinding by using a grinder, and controlling the grinding particle size D50 of the slurry to be 0.45 um; thirdly, spray drying is carried out; fourthly, roasting the dried powder for 8 hours at 700 ℃ under the nitrogen protection atmosphere to obtain a lithium iron phosphate sample; preparing lithium oxalate lithium supplementing slurry: weighing 5g of deionized water, adding the deionized water into a beaker, weighing 0.25g of CMC, adding the CMC into the deionized water, stirring and dissolving, simultaneously weighing 2.5g of lithium oxalate, 1g of LNMO and 1g of carbon black, grinding and mixing, adding the mixture into the solution, stirring until the mixture is uniformly dispersed, finally weighing 1g of SBR (with the solid content of 25 percent), adding the SBR into the dispersion, and continuously stirring until uniform slurry is obtained. Sixthly, mixing the following components in percentage by mass of 100: 1: 2.5: 100, respectively and uniformly stirring and mixing a lithium iron phosphate positive active material with a conductive agent (CNT: graphene =5: 5), PVDF and NMP to obtain a positive slurry, coating the positive slurry on one surface of an Al foil, drying the Al foil in vacuum at 110 ℃, pressing the Al foil into a wafer with the thickness of less than 0.3mm and phi 12mm as a positive electrode, weighing, calculating the mass of lithium iron phosphate in the positive electrode wafer, and then according to the weight ratio of the lithium iron phosphate: and (4) weighing lithium oxalate slurry with the corresponding mass and coating the lithium oxalate slurry on a positive plate, wherein the lithium oxalate slurry is lithium oxalate =100: 4. And then, placing the pole piece in a vacuum oven, baking for 6 hours at 120 ℃ to be used as a positive pole, assembling the positive pole, a metal lithium piece, a diaphragm and electrolyte into a button cell, and then carrying out charge and discharge tests.
Specific capacity test
Using the above comparative examples and examples to prepare a positive electrode sheet as a positive electrode, a metallic lithium sheet as a negative electrode, a Celgard 2300 microporous membrane as a separator, 1.0mol/L Ethylene Carbonate (EC) of LiPF 6: dimethyl carbonate (DMC) =1: 1-5 (volume ratio) solution is used as electrolyte, and the R2025 button cell is assembled in a glove box. Electrochemical performance tests were performed as follows using the nover 3008 cell test system.
Specific capacity test: firstly, the battery is charged and discharged once at the normal temperature of 25 ℃ at 0.1C, and the charge and discharge cut-off voltage is 2.5-3.8V (vs. Li/Li +). The specific capacity data is converted according to the mass of the active material lithium iron phosphate in the positive plate, and the charging and discharging specific capacity and the discharging efficiency are shown in the following table 1.
TABLE 1 specific capacity test results
Experiment number Specific capacity for first charge (mAh/g) Specific capacity of first discharge (mAh/g) Efficiency of discharge
Comparative example 1 160.0 159.8 99.88%
Comparative example 2 169.6 160 94.34%
Example 1 163.2 159.9 98.01%
Example 2 174.6 160.1 91.70%
In the comparative examples and examples, the negative electrode used was a lithium sheet, and the specific discharge capacity was about 160mAh/g, which was limited by the positive electrode material. Comparative example 1 is that lithium oxalate was added directly at the battery stage, and the lithium iron phosphate system did not decompose at low voltage and without catalyst, and it did not increase the charge capacity; when the coating amount of lithium oxalate is 1% and 4%, respectively, the charging capacities of the embodiment 1 and the embodiment 2 are respectively improved by 3.2mAh/g and 14.6mAh/g compared with the comparative example 1, while the lithium oxalate in the comparative example 1 is added into the battery slurry independently, and the capacity is not improved because the decomposition potential is low and the lithium oxalate is not decomposed; in the comparative example 2, the addition ratio of lithium oxalate is 4% (more than 1% of the example 1), lithium oxalate, a catalyst and a carbon source are prepared into lithium supplement slurry, and then the lithium supplement slurry is coated on the surface layer, so that the lithium oxalate is partially decomposed due to no nanocrystallization and in-situ carbon coating, compared with the comparative example 1, the capacity is improved by 9.6mAh/g, and the capacity improvement degree is not higher than that of the example 2. In addition, when the coating capacity of the lithium oxalate is 4%, the charging capacity is improved by more than 9%, lithium consumed by the generation of the first negative electrode graphite SEI film can be completely offset, and the coating capacity of the lithium oxalate is continuously improved, so that the discharging capacity of the battery cannot be continuously improved due to the limitation of the specific capacity of the positive electrode, and the discharging specific capacity of the positive electrode cannot be continuously improved.
While the foregoing is directed to embodiments of the present invention, it will be understood by those skilled in the art that various changes may be made without departing from the spirit and scope of the invention.

Claims (12)

1. The modified lithium iron phosphate composite positive electrode material for the lithium ion battery is characterized by comprising an inner core and a coating layer coated on the outer surface of the inner core, wherein the inner core comprises carbon and lithium iron phosphate, and the coating layer comprises carbon and lithium oxalate; the mass ratio of the carbon in the core, the carbon in the coating layer, the lithium oxalate in the coating layer and the core is (1.95-3): 0.5-1.0: 1.0-4: 92-100 parts; the preparation method of the modified lithium iron phosphate lithium ion battery composite positive electrode material comprises the following steps: s1, adding a lithium source, an iron source, a phosphorus source and a first carbon source into the first solvent, and mixing to obtain a first mixture; s2, grinding the first mixture by using a grinder to obtain first slurry; s3, spray drying the first slurry to obtain first powder; s4, roasting the first powder in a protective gas atmosphere to obtain carbon-coated lithium iron phosphate, wherein the protective gas is one or more of argon, helium, nitrogen and carbon dioxide; s5, adding lithium oxalate, the carbon-coated lithium iron phosphate and a second carbon source into a second solvent for mixing to obtain a second mixture; s6, spray drying the second mixture to obtain second powder; and S7, roasting the second powder under the protective gas atmosphere to obtain the modified lithium iron phosphate lithium ion battery composite positive electrode material, wherein the protective gas is one or more of argon, helium, nitrogen and carbon dioxide.
2. The modified lithium iron phosphate lithium ion battery composite positive electrode material as claimed in claim 1, wherein the core is carbon-coated doped lithium iron phosphate, the lithium iron phosphate in the core is doped lithium iron phosphate, and the general formula of the core is LiFeγM1-γPO4and/C, wherein M is one or more of vanadium V, niobium Nb, titanium Ti, tungsten W, lanthanum La, yttrium Y, zirconium Zr or magnesium Mg, and gamma is more than or equal to 0.8 and less than 1.
3. The modified lithium iron phosphate lithium ion battery composite positive electrode material as claimed in claim 1, wherein the core is carbon-coated doped lithium iron phosphate, the lithium iron phosphate in the core is doped lithium iron phosphate, and the general formula of the core is LiFeαM1-αPO4and/C, wherein M is manganese Mn, and alpha is more than or equal to 0.2 and less than 1.
4. A method for preparing the modified lithium iron phosphate composite positive electrode material of any one of claims 1 to 3, comprising: s1, adding a lithium source, an iron source, a phosphorus source and a first carbon source into the first solvent, and mixing to obtain a first mixture; s2, grinding the first mixture by using a grinder to obtain first slurry; s3, spray drying the first slurry to obtain first powder; s4, roasting the first powder in a protective gas atmosphere to obtain carbon-coated lithium iron phosphate, wherein the protective gas is one or more of argon, helium, nitrogen and carbon dioxide; s5, adding lithium oxalate, the carbon-coated lithium iron phosphate and a second carbon source into a second solvent for mixing to obtain a second mixture; s6, spray drying the second mixture to obtain second powder; s7, roasting the second powder under the protective gas atmosphere to obtain the modified lithium iron phosphate lithium ion battery composite positive electrode material, wherein the protective gas is one or more selected from argon, helium, nitrogen and carbon dioxide; wherein the mass ratio of the residual amount of the carbon element in the first carbon source after roasting to the carbon-coated lithium iron phosphate is (1.95-3): (92-100); the ratio of the mass of the lithium oxalate to the mass of the carbon-coated lithium iron phosphate is (1.0-4): (92-100); the mass ratio of the residual amount of the carbon element in the second carbon source after roasting to the carbon-coated lithium iron phosphate is (0.5-1.0): (92-100).
5. The method for preparing the modified lithium iron phosphate lithium ion battery composite positive electrode material according to claim 4, wherein the carbon-coated lithium iron phosphate is carbon-coated doped lithium iron phosphate; step S1 specifically includes: adding a lithium source, an iron source, a doped metal source, a phosphorus source and a first carbon source into a first solvent, and mixing to obtain a first mixture, wherein the molar ratio of lithium in the lithium source, iron in the iron source, the doped metal in the doped metal source and phosphorus in the phosphorus source is 1: γ: (1-. gamma.): 1, wherein gamma is more than or equal to 0.8 and less than 1, and the doped metal is one or more of vanadium V, niobium Nb, titanium Ti, tungsten W, lanthanum La, yttrium Y, zirconium Zr or magnesium Mg.
6. The method for preparing the modified lithium iron phosphate lithium ion battery composite positive electrode material according to claim 4, wherein the carbon-coated lithium iron phosphate is carbon-coated doped lithium iron phosphate; step S1 specifically includes: adding a lithium source, an iron source, a doped metal source, a phosphorus source and a first carbon source into a first solvent, and mixing to obtain a first mixture, wherein the molar ratio of lithium in the lithium source, iron in the iron source, the doped metal in the doped metal source and phosphorus in the phosphorus source is 1: α: (1-. alpha.): 1, wherein alpha is more than or equal to 0.2 and less than 1, and the doping metal in the doping metal source is manganese Mn.
7. The preparation method of the modified lithium iron phosphate composite positive electrode material for the lithium ion battery according to claim 4, wherein the roasting temperature of the first powder is 650-795 ℃; the roasting temperature of the second powder is 350-450 ℃; and/or the grinding particle size D50 of the first slurry is 0.25-0.5 μm.
8. The method for preparing the modified lithium iron phosphate composite positive electrode material for the lithium ion battery according to claim 4, wherein the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium citrate and lithium dihydrogen phosphate; the iron source is selected from one or more of ferric oxide, ferroferric oxide, ferric citrate, ferric phosphate and ferrous oxalate; and/or the phosphorus source is selected from one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid and lithium dihydrogen phosphate; and/or the first carbon source or the second carbon source is selected from one or more of glucose, sucrose, citric acid, polyethylene glycol, polyvinyl butyral, polyvinyl alcohol, polyacrylonitrile, starch, cellulose; and/or the first solvent is an organic solvent or water, the organic solvent is selected from one of methanol, ethanol, acetone and N-methyl-2-pyrrolidone, and the second solvent is water.
9. The method for preparing the modified lithium iron phosphate composite positive electrode material for the lithium ion battery according to claim 5, wherein the doped metal source is magnesium oxalate.
10. The method for preparing the modified lithium iron phosphate lithium ion battery composite positive electrode material according to claim 6, wherein the doped metal source is trimanganese tetroxide.
11. A lithium ion battery positive electrode is characterized by comprising a modified lithium iron phosphate lithium ion battery composite positive electrode material, a conductive agent, PVDF and NMP, wherein the modified lithium iron phosphate lithium ion battery composite positive electrode material is the modified lithium iron phosphate lithium ion battery composite positive electrode material according to any one of claims 1 to 3 or is prepared according to the preparation method of any one of claims 4 to 10.
12. A preparation method of a lithium ion battery anode is characterized by comprising the following steps: stirring and mixing the modified lithium iron phosphate lithium ion battery composite positive electrode material, a conductive agent, PVDF and NMP to obtain positive electrode slurry; coating the positive electrode slurry on an aluminum foil to obtain a positive electrode; drying the positive electrode under vacuum, and pressing the dried positive electrode into a sheet shape, wherein the thickness of the sheet positive electrode is less than 0.3 mm; the modified lithium iron phosphate composite positive electrode material for the lithium ion battery is prepared according to the preparation method of any one of claims 4 to 10, and is the modified lithium iron phosphate composite positive electrode material for the lithium ion battery according to any one of claims 1 to 3.
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