CN114530594A - High-conductivity long-cycle lithium iron phosphate battery and preparation method thereof - Google Patents
High-conductivity long-cycle lithium iron phosphate battery and preparation method thereof Download PDFInfo
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Abstract
The invention relates to the field of lithium ion battery materials, and discloses a high-conductivity long-cycle lithium iron phosphate battery and a preparation method thereof, wherein the lithium iron phosphate battery comprises a positive plate, a negative plate, a diaphragm and electrolyte, wherein the positive plate is coated with positive slurry comprising reduced graphene oxide-aluminum modified lithium iron phosphate, and the preparation method comprises the following steps: (1) preparing a positive plate: preparing reduced graphene oxide-aluminum modified lithium iron phosphate, coating positive electrode slurry containing the reduced graphene oxide-aluminum modified lithium iron phosphate on the surface of a current collector, and cutting and baking to obtain a positive electrode piece; (2) preparing a negative plate; (3) assembling a battery; (4) and (4) forming and grading. The conductivity of the battery positive plate is enhanced by reducing the graphene oxide-aluminum modified lithium iron phosphate positive material; a proper electrolyte system is selected to improve the long-cycle capacity retention rate; the formation process is optimized, the thickness of SEI films generated on the positive electrode and the negative electrode is controlled, and the obtained lithium iron phosphate battery has good working performance at normal temperature and high temperature.
Description
Technical Field
The invention relates to the field of lithium ion battery materials, in particular to a high-conductivity long-cycle lithium iron phosphate battery and a preparation method thereof.
Background
With the development of science and technology, lithium batteries have the advantages of stable discharge voltage, wide working temperature range, low self-discharge rate, cyclic charge and discharge, long storage life and the like, have gradually become mainstream, and have wide application range. The anode material of the lithium battery is an important component, and the currently commonly used anode materials include LCo (lithium cobaltate), LMO (lithium manganate), NCM (ternary system), NCA (binary system), LFP (lithium iron phosphate), and the like. The lithium iron phosphate material has rich raw material sources, is environment-friendly, has high safety performance and thermal stability, has good cycle performance, and is developed very quickly. The main problems of the current lithium iron phosphate anode material are low ion diffusion coefficient and low electronic conductivity.
The method for improving the conductivity of the lithium iron phosphate material is commonly used at present, metal ions or carbon sources are introduced into the material, and Chinese patent publication No. CN106898760A discloses a modified lithium iron phosphate material of the lithium iron phosphate lithium battery, which comprises modified lithium iron phosphate doped with 0.1% by mass of titanium, 0.2% by mass of magnesium and 0.1% by mass of silver, and the modified lithium iron phosphate material is uniformly mixed to obtain a positive active substance, so that the conductivity and tap density of the positive material of the lithium iron phosphate lithium battery are improved, the stability of the generating voltage of the battery is improved, and the service life of the battery is prolonged. Chinese patent publication No. CN109301195A discloses a high-conductivity lithium iron phosphate material and a preparation method thereof, wherein the high-conductivity lithium iron phosphate material is obtained by sintering lithium iron phosphate, graphene slurry, vapor-grown carbon fiber VGCF or carbon nanotubes and a precursor containing noble metal at high temperature. The high conductivity of the graphene, the bridge function of the carbon fiber and the extremely high conductivity of the noble metal are comprehensively utilized, so that the high-rate discharge performance of the lithium iron phosphate battery system is greatly improved. However, in these modification methods, the dispersibility of graphene is general, and deposition is likely to occur in the coating process, which affects the action effect of graphene and lithium iron phosphate.
Disclosure of Invention
The invention aims to solve the problems of low conductivity and low capacity retention rate after repeated circulation of a positive electrode of a lithium iron phosphate battery in the prior art, and provides a high-conductivity long-circulation lithium iron phosphate battery and a preparation method thereof.
In order to achieve the purpose, the invention adopts the following technical scheme:
one of the purposes of the invention is to provide a high-conductivity long-cycle lithium iron phosphate battery, which comprises a positive plate, a negative plate, a diaphragm and electrolyte, wherein the positive plate is a carbon-coated aluminum foil coated with positive electrode slurry comprising reduced graphene oxide-aluminum modified lithium iron phosphate.
The reduced graphene oxide-aluminum modified lithium iron phosphate is used in the anode slurry, the reduced graphene oxide-aluminum complex is prepared by utilizing the process of reducing graphene oxide with aluminum, and the reduced graphene oxide-aluminum complex is added into the lithium iron phosphate anode material in the preparation process, so that the reduced graphene oxide-aluminum complex is doped into the lithium iron phosphate, the conductivity of the obtained lithium iron phosphate anode material is enhanced, the solvent infiltration characteristic of the lithium iron phosphate anode material is improved, the lithium iron phosphate anode material has smaller internal resistance and higher working efficiency in the working process, and the cycle characteristic of the battery is improved.
Preferably, the positive electrode slurry further comprises a conductive agent and a binder, and the components are calculated according to the following parts by weight: 90-95 parts of reduced graphene oxide-aluminum modified lithium iron phosphate, 1-5 parts of a conductive agent and 1-3 parts of an adhesive.
Preferably, the negative plate is a copper foil coated with negative slurry, and the negative slurry comprises a negative material, a conductive agent and a binder; the negative electrode material is one of natural graphite, artificial graphite, hard carbon, soft carbon and lithium metal.
Preferably, the conductive agent is one or a combination of carbon nanotubes, ketjen black, acetylene black and conductive carbon black, and the binder is one of CMC, polyvinylidene fluoride and polyvinyl alcohol.
Preferably, the electrolyte includes a lithium salt, an additive, and a solvent; the lithium salt is 1-1.3 mol/L lithium hexafluorophosphate, the solvent is a composition of at least two of ethylene carbonate, propylene carbonate, ethyl acetate and ethyl propionate, and the additive is one of chloroethylene, ethylene sulfate, vinylene carbonate, ethylene carbonate and diethyl carbonate.
By compounding the electrolyte solvent, better working efficiency is achieved; the addition of the additive can improve the capacity retention rate of the lithium iron phosphate battery in an extreme environment and prolong the service life of the lithium iron phosphate battery.
The invention also aims to provide a preparation method of the high-conductivity long-cycle lithium iron phosphate battery, which comprises the following steps:
(1) preparing a positive plate: preparing reduced graphene oxide-aluminum modified lithium iron phosphate, coating the positive electrode slurry containing the reduced graphene oxide-aluminum modified lithium iron phosphate on the surface of a carbon-coated aluminum foil, and cutting and baking to obtain a positive electrode plate;
(2) preparing a negative plate: mixing a negative electrode material, a binder and a conductive agent to obtain a negative electrode slurry, coating the negative electrode slurry on the surface of copper foil, and cutting and baking to obtain a negative electrode plate;
(3) assembling the battery: assembling a positive pole piece, a negative pole piece, a diaphragm and electrolyte;
(4) formation and capacity grading: the formation steps are as follows: charging the battery for 60-90 min by using a current of 0.05-0.1C, standing the battery for 24-48 h at 35-38 ℃, charging the battery to 3.3-3.65V by using a current of 0.2-0.5C, and then carrying out capacity grading to obtain the high-conductivity long-cycle lithium iron phosphate battery.
Preferably, the battery assembling step comprises rolling, slitting, sheet making, winding, assembling, top side sealing, drying and electrolyte injection.
Preferably, the formation step is: charging the battery for 60-90 min by using a current of 0.05-0.1 ℃, standing the battery for 24-48 h at 35-38 ℃, and charging the battery to 3.3-3.65V by using a current of 0.2-0.5 ℃.
The purpose of formation is mainly as follows: firstly, active substances in the battery are converted into substances with normal electrochemical action by virtue of first charging; and secondly, a uniform SEI film is formed on the surfaces of the carbon material of the cathode of the electrode and the lithium iron phosphate material of the anode. In the formation process, the battery is charged for a period of time by a small current, so that a uniform SEI film can be formed on the surfaces of a positive electrode and a negative electrode, and then polarization is eliminated by standing. If the charging current is too large or the charging current is too high, the electrolyte may be decomposed too much during formation, so that excessive gas may be generated, and the battery may swell. If the standing time is too short, the surface of the positive electrode graphene modified lithium iron phosphate material cannot absorb enough electrolyte, which may cause a serious polarization phenomenon and affect the service life and work of the battery. Through multiple experiments, the formation charging and discharging parameters are optimized, so that the prepared lithium iron phosphate battery has good working performance.
Preferably, the preparation method of the reduced graphene oxide-aluminum modified lithium iron phosphate comprises the following steps:
(a) dissolving graphene oxide in water, ultrasonically dispersing for 1-4 hours to prepare a solution with the mass fraction of 0.5-1%, adding an HCl solution with the mass fraction of 30-35%, wherein the volume ratio of the graphene oxide solution to the HCl solution is 3-5: 1, adding aluminum powder with the mass of 4-6 times that of the graphene oxide, and reacting for 30-60 min to prepare a reduced graphene oxide-aluminum solution;
(b) adding an iron source, a phosphorus source and a lithium source into the reduced graphene oxide-aluminum solution, wherein the mass ratio of the total mass of the iron source, the phosphorus source and the lithium source to the mixed solution is (0.5-2): 1, uniformly mixing, heating to 60-80 ℃, and reacting for 2-4 hours;
(c) drying at 120-180 ℃ for 12-18 h, and sintering at 600-900 ℃ for 8-12 h to obtain the reduced graphene oxide-aluminum modified lithium iron phosphate cathode material.
In the step (a), firstly, graphene oxide is dispersed into a single-layer state through an ultrasonic effect, then hydrochloric acid and aluminum powder are added into the graphene oxide, and reduction reaction is carried out on the graphene oxide under a birch reduction mechanism by utilizing high reducibility of nascent hydrogen obtained by the reaction of the hydrochloric acid and the aluminum powder, so that most oxygen-containing functional groups in the graphene oxide are reduced, and the structure of the graphene is recovered. Meanwhile, aluminum ions generated in the reduction process can also be retained in the reduced graphene oxide system to form a reduced graphene oxide-aluminum complex.
In the step (b), the reduced graphene oxide-aluminum complex is added into a preparation raw material of the lithium iron phosphate anode material, and a certain amount of aluminum ions exist in the reduced graphene oxide structure, so that the agglomeration phenomenon can be effectively inhibited, meanwhile, the addition of the aluminum ions can also increase the lattice defect of the material, expand the diffusion channel of the lithium ions and reduce the insertion/extraction resistance of the lithium ions, thereby being beneficial to improving the ionic and electronic conductivity of the lithium iron phosphate and improving the conductivity of the lithium iron phosphate anode material. In addition, the reduced graphene oxide-aluminum composite is of a single-layer structure, so that the dispersibility of the reduced graphene oxide in the cathode material can be greatly improved.
Through the control of the high-temperature sintering condition in the step (c), the unreduced graphene oxide possibly existing in the step (1) can be continuously reduced into graphene at high temperature, the conductivity of the material is further improved, meanwhile, as partial gas is generated, the pore structure in the material is increased, the specific surface area of the material can be increased, and the infiltration of electrolyte is facilitated. The reduced graphene oxide is utilized to ensure that the introduction of aluminum ions can be more uniform. Meanwhile, in the high-temperature sintering process, aluminum ions in the reduced graphene oxide-aluminum ion compound can play a role in catalysis, and carbon atoms at specific points are removed, so that partial holes appear in the carbon atom structure in the graphene material, and due to the increase of the activity of the edge of the hole, the conductivity of the graphene material can be improved, and the conductivity of the positive electrode material is further improved.
Preferably, the iron source is one of iron powder, ferrous oxalate, ferrous sulfate and ferrous nitrate; the phosphorus source is one of phosphoric acid and ammonium dihydrogen phosphate; the lithium source is one of lithium carbonate, lithium acetate and lithium hydroxide; the ratio of the amounts of the iron source, the phosphorus source and the lithium source is (0.95-1.05): (0.9-1.1): 0.95-1.05).
Therefore, the invention has the following beneficial effects: (1) in the preparation process of the lithium iron phosphate battery, a reduced graphene oxide-aluminum modified lithium iron phosphate positive electrode material is used in the positive electrode coating, and the conductivity of the battery positive electrode plate is enhanced by utilizing the good conductivity and porous structure of the material; (2) through comparing and screening electrolytes with different performance parameters and wettability, evaluating the influence of different electrolytes on the cycle electrochemical performance of the battery, particularly the capacity retention rate, selecting a proper electrolyte system, and improving the long-cycle capacity retention rate of the battery; (3) through multiple experiments, the formation charge and discharge parameters are optimized, and the thickness of SEI films generated on the positive electrode and the negative electrode is controlled, so that the prepared lithium iron phosphate battery has good working performance at normal temperature and high temperature.
Detailed Description
The invention is further described with reference to specific embodiments. It is to be understood that these examples are suitable for illustrating the basic features and advantages of the invention, and the invention is not to be limited in scope by the following examples; the implementation conditions used in the examples can be further adjusted according to specific requirements, and the implementation conditions not indicated are generally the conditions used in routine experiments.
Not specifically illustrated in the following examples, all starting materials are commercially available or prepared by methods conventional in the art.
Example 1
A preparation method of a high-conductivity long-cycle lithium iron phosphate battery comprises the following steps:
(1) preparing a positive plate: preparing reduced graphene oxide-aluminum modified lithium iron phosphate, and then sanding the prepared reduced graphene oxide-aluminum modified lithium iron phosphate to enable D50 to be less than or equal to 2 microns. And then mixing and homogenizing 93 parts by weight of reduced graphene oxide-aluminum modified lithium iron phosphate, 3 parts by weight of acetylene black, 2 parts by weight of carbon nanotubes and 2 parts by weight of polyvinylidene fluoride at 40 ℃ to obtain the anode coating with uniform particles and no bubbles. Coating the positive electrode slurry containing reduced graphene oxide-aluminum modified lithium iron phosphate on the surface of a carbon-coated aluminum foil, wherein the coating density is 10mg/cm2Cutting and baking to obtain a positive pole piece;
(2) preparing a negative plate: mixing 90 parts of artificial graphite, 3 parts of CMC and 2 parts of conductive carbon black in parts by weightHomogenizing to obtain negative electrode slurry, coating the negative electrode slurry on the surface of copper foil with a coating density of 10mg/cm2Cutting and baking to obtain a negative pole piece;
(3) assembling the battery: rolling, cutting, flaking and winding the positive pole piece and the negative pole piece, assembling, top sealing, drying and injecting electrolyte, wherein the electrolyte is 1.2mol/L lithium hexafluorophosphate, the solvent is a composition of ethylene carbonate, ethyl propionate and propylene carbonate with the mass ratio of 30:65:5, and vinylene carbonate with the mass fraction of 1% is added;
(4) formation and capacity grading: charging the battery for 75min by using a current of 0.07C, standing the battery for 36h at 38 ℃, and then charging the battery to 2.45V by using a current of 0.3C; in the capacity grading process, charging the battery to 2.6V by using a current of 0.1C, and standing for 45 min; charging the battery to 3.3V by using a current of 0.3C, and standing for 10 min; discharging the battery to 2.2V by using a current of 0.7C, and standing for 10 min; and then charging the battery to 35% of the capacity by using a current of 0.7C to obtain the high-conductivity lithium iron phosphate battery.
The preparation method of the reduced graphene oxide-aluminum modified lithium iron phosphate comprises the following steps:
(a) dissolving graphene oxide in water, performing ultrasonic dispersion for 3 hours at 80kHz to prepare a solution with the mass fraction of 0.8%, adding 35% of HCl solution, wherein the volume ratio of the graphene oxide solution to the HCl solution is 4:1, adding aluminum powder (the particle size is less than 10 mu m) which is 5 times of the mass of the graphene oxide and 0.1g/L of surfactant sodium dodecyl sulfate, and reacting for 45 minutes to prepare a reduced graphene oxide-aluminum solution;
(b) adding ferrous oxalate, phosphoric acid and lithium carbonate into the reduced graphene oxide-aluminum solution, wherein the mass ratio of the ferrous oxalate, the phosphoric acid and the lithium carbonate is 1:1:1, the mass ratio of the total mass of the ferrous oxalate, the phosphoric acid and the lithium carbonate to the mass of the mixed solution is 1:1, uniformly mixing, adding ascorbic acid with the mass fraction of 0.3%, heating to 70 ℃, and reacting for 3 hours;
(c) and under the nitrogen atmosphere, heating to 150 ℃ at the heating rate of 5 ℃/min, drying for 12h, heating to 800 ℃ at the heating rate of 5 ℃/min, and sintering at high temperature for 10h to obtain the graphene modified lithium iron phosphate cathode material.
Example 2
A preparation method of a high-conductivity long-cycle lithium iron phosphate battery comprises the following steps:
(1) preparing a positive plate: preparing reduced graphene oxide-aluminum modified lithium iron phosphate, and then sanding the prepared reduced graphene oxide-aluminum modified lithium iron phosphate to enable D50 to be less than or equal to 2 microns. And then mixing and homogenizing 90 parts by weight of reduced graphene oxide-aluminum modified lithium iron phosphate, 5 parts by weight of Ketjen black and 3 parts by weight of polyvinyl alcohol at 40 ℃ to obtain the positive coating with uniform particles and no bubbles. Coating the positive electrode slurry containing reduced graphene oxide-aluminum modified lithium iron phosphate on the surface of a carbon-coated aluminum foil, wherein the coating density is 10mg/cm2Cutting and baking to obtain a positive pole piece;
(2) preparing a negative plate: mixing and homogenizing 95 parts of artificial graphite, 5 parts of CMC and 2 parts of acetylene black in parts by weight to obtain negative electrode slurry, coating the negative electrode slurry on the surface of copper foil, wherein the coating density is 10mg/cm2Cutting and baking to obtain a negative pole piece; (3) assembling the battery: rolling, slitting, flaking and winding the positive pole piece and the negative pole piece, assembling, top side sealing, drying and injecting electrolyte, wherein the electrolyte is 1mol/L lithium hexafluorophosphate, the solvent is a composition of ethyl acetate and ethylene carbonate with a mass ratio of 45:55, and diethyl carbonate with a mass fraction of 2% is added;
(4) formation and capacity grading: charging the battery for 90min by using a current of 0.05C, standing the battery for 48h at 38 ℃, and then charging the battery to 2.3V by using a current of 0.2C; in the capacity grading process, charging the battery to 2.3V by using 0.05C current, and standing for 30 min; charging the battery to 2.8V by using a current of 0.2C, and standing for 5 min; discharging the battery to 2.0V by using a current of 0.2C, and standing for 5 min; and then charging the battery to 40% of the capacity by using 0.5C current to obtain the high-conductivity lithium iron phosphate battery.
The preparation method of the reduced graphene oxide-aluminum modified lithium iron phosphate comprises the following steps:
(a) dissolving graphene oxide in water, performing ultrasonic dispersion for 1h at 100kHz to prepare a solution with the mass fraction of 0.5%, adding a HCl solution with the mass fraction of 30%, wherein the volume ratio of the graphene oxide solution to the HCl solution is 3:1, adding aluminum powder (the particle size is less than 10 mu m) which is 4 times of the mass of the graphene oxide and 0.01g/L of surfactant lithium dodecyl sulfate, and reacting for 30min to prepare a reduced graphene oxide-aluminum solution;
(b) adding iron powder, phosphoric acid and lithium acetate into the reduced graphene oxide-aluminum solution, wherein the weight ratio of the iron powder, the phosphoric acid and the lithium acetate is 0.95:1:1, the total weight ratio of the iron powder, the phosphoric acid and the lithium acetate to the mixed solution is 2:1, uniformly mixing, adding 0.1% by mass of citric acid, heating to 60 ℃, and reacting for 4 hours;
(c) and under the nitrogen atmosphere, heating to 180 ℃ at the heating rate of 5 ℃/min, drying for 12h, heating to 600 ℃ at the heating rate of 5 ℃/min, and sintering at high temperature for 12h to obtain the graphene modified lithium iron phosphate cathode material.
Example 3
A preparation method of a high-conductivity long-cycle lithium iron phosphate battery comprises the following steps:
(1) preparing a positive plate: preparing reduced graphene oxide-aluminum modified lithium iron phosphate, and then sanding the prepared reduced graphene oxide-aluminum modified lithium iron phosphate to enable D50 to be less than or equal to 2 microns. And then mixing and homogenizing 95 parts by weight of reduced graphene oxide-aluminum modified lithium iron phosphate, 1 part by weight of conductive carbon black and 1 part by weight of polyvinylidene fluoride at 40 ℃ to obtain the positive coating with uniform particles and no bubbles. Coating the positive electrode slurry containing reduced graphene oxide-aluminum modified lithium iron phosphate on the surface of a carbon-coated aluminum foil, wherein the coating density is 10mg/cm2Cutting and baking to obtain a positive pole piece;
(2) preparing a negative plate: mixing and homogenizing 90 parts by weight of graphite, 4 parts by weight of CMC and 5 parts by weight of carbon nano tube to obtain negative electrode slurry, coating the negative electrode slurry on the surface of copper foil, wherein the coating density is 10mg/cm2Cutting and baking to obtain a negative pole piece; (3) assembling the battery: rolling, slitting, sheet-making and winding the positive pole piece and the negative pole piece, assembling, top side sealing, drying and injecting electrolyte, wherein the electrolyte is 1.3mol/L lithium hexafluorophosphate, and the solvent is ethyl propionate and ethylene carbonate with the mass ratio of 35:60:5And propylene carbonate, and 1.5 percent of ethylene sulfate is added;
(4) formation and capacity grading: charging the battery for 60min by using a current of 0.1C, standing the battery for 24h at 35 ℃, and charging the battery to 2.65V by using a current of 0.5C; in the capacity grading process, charging the battery to 3.0V by using a current of 0.1C, and standing for 60 min; charging the battery to 3.65V by using 0.5C current, and standing for 10 min; discharging the battery to 2.4V by using a current of 0.5C, and standing for 10 min; and then charging the battery to 30% of the capacity by using the current of 1C to obtain the high-conductivity lithium iron phosphate battery.
The preparation method of the reduced graphene oxide-aluminum modified lithium iron phosphate comprises the following steps:
(a) dissolving graphene oxide in water, performing ultrasonic dispersion for 4 hours at 30kHz to prepare a solution with the mass fraction of 1%, adding 35% HCl solution, wherein the volume ratio of the graphene oxide solution to the HCl solution is 5:1, adding aluminum powder (the particle size is less than 10 microns) which is 6 times of the mass of the graphene oxide, and 0.3g/L surfactant of magnesium dodecyl sulfate and sodium dodecyl sulfate, and reacting for 60 minutes to prepare a reduced graphene oxide-aluminum solution;
(b) adding ferrous sulfate, ammonium dihydrogen phosphate and lithium hydroxide into the reduced graphene oxide-aluminum solution, wherein the mass ratio of the ferrous sulfate, the ammonium dihydrogen phosphate and the lithium hydroxide is 1.05:1:1.05, the mass ratio of the total mass of the ferrous sulfate, the ammonium dihydrogen phosphate and the lithium hydroxide to the mixed solution is 0.5:1, uniformly mixing, adding oxalic acid with the mass fraction of 0.5%, heating to 80 ℃, and reacting for 2 hours;
(c) and under the nitrogen atmosphere, heating to 120 ℃ at the heating rate of 5 ℃/min, drying for 18h, heating to 900 ℃ at the heating rate of 5 ℃/min, and sintering at high temperature for 8h to obtain the graphene modified lithium iron phosphate cathode material.
Example 4
A preparation method of a high-conductivity long-cycle lithium iron phosphate battery comprises the following steps:
(1) preparing a positive plate: preparing reduced graphene oxide-aluminum modified lithium iron phosphate by the same method as in example 1, and modifying the prepared reduced graphene oxide-aluminumAnd sanding the lithium iron phosphate to ensure that D50 is less than or equal to 2 mu m. And then mixing and homogenizing 90 parts by weight of reduced graphene oxide-aluminum modified lithium iron phosphate, 3 parts by weight of conductive carbon black, 2 parts by weight of carbon nano tube and 5 parts by weight of polyvinylidene fluoride at 40 ℃ to obtain the anode coating with uniform particles and no bubbles. Coating the positive electrode slurry containing the reduced graphene oxide-aluminum modified lithium iron phosphate on the surface of a carbon-coated aluminum foil, wherein the coating density is 10mg/cm2Cutting and baking to obtain a positive pole piece;
(2) preparing a negative plate: mixing and homogenizing 85 parts of graphite, 5 parts of CMC, 5 parts of acetylene black and 2 parts of conductive carbon black in parts by weight to obtain negative electrode slurry, coating the negative electrode slurry on the surface of copper foil, wherein the coating density is 10mg/cm2Cutting and baking to obtain a negative pole piece;
(3) assembling the battery: rolling, cutting, slicing and winding the positive pole piece and the negative pole piece, assembling, top sealing, drying, injecting electrolyte, wherein the electrolyte is 1.1mol/L lithium hexafluorophosphate, the solvent is a composition of ethyl acetate and ethylene carbonate with the mass ratio of 40:60, and ethylene carbonate with the mass fraction of 2% is added;
(4) formation and capacity grading: charging the battery for 70min by using a current of 0.06C, standing the battery for 30h at 36 ℃, and charging the battery to 2.4V by using a current of 0.35C; in the capacity grading process, charging the battery to 2.4V by using a current of 0.2C, and standing for 60 min; charging the battery to 3.45V by using a current of 0.4C, and standing for 10 min; discharging the battery to 2.1V by using a current of 0.6C, and standing for 10 min; and then charging the battery to 30% of the capacity by using the current of 0.8C to obtain the high-conductivity lithium iron phosphate battery.
Example 5
A preparation method of a high-conductivity long-cycle lithium iron phosphate battery comprises the following steps:
(1) preparing a positive plate: the reduced graphene oxide-aluminum modified lithium iron phosphate is prepared by the same method as that in the embodiment 1, and then the prepared reduced graphene oxide-aluminum modified lithium iron phosphate is subjected to sand milling, so that D50 is less than or equal to 2 microns. Then, 95 parts by weight of reduced graphene oxide-aluminum modified lithium iron phosphate and 1 part by weight of conductive carbonAnd mixing and homogenizing the black and 1 part of polyvinylidene fluoride at 40 ℃ to obtain the anode coating with uniform particles and no air bubbles. Coating the positive electrode slurry containing reduced graphene oxide-aluminum modified lithium iron phosphate on the surface of a carbon-coated aluminum foil, wherein the coating density is 10mg/cm2Cutting and baking to obtain a positive pole piece;
(2) preparing a negative plate: mixing and homogenizing 90 parts of graphite, 4 parts of CMC and 5 parts of carbon nano tube in parts by weight to obtain negative electrode slurry, coating the negative electrode slurry on the surface of copper foil, wherein the coating density is 10mg/cm2Cutting and baking to obtain a negative pole piece; (3) assembling the battery: rolling, cutting, flaking and winding the positive pole piece and the negative pole piece, assembling, top sealing, drying and injecting electrolyte, wherein the electrolyte is 1.3mol/L lithium hexafluorophosphate, the solvent is a composition of ethyl propionate, ethylene carbonate and propylene carbonate with the mass ratio of 35:60:5, and 1.5% of ethylene sulfate is added;
(4) formation and capacity grading: charging the battery for 70min by using a current of 0.06C, standing the battery for 30h at 36 ℃, and charging the battery to 2.4V by using a current of 0.35C; in the capacity grading process, charging the battery to 2.4V by using a current of 0.2C, and standing for 60 min; charging the battery to 3.45V by using a current of 0.4C, and standing for 10 min; discharging the battery to 2.1V by using a current of 0.6C, and standing for 10 min; and then charging the battery to 30% of the capacity by using the current of 0.8C to obtain the high-conductivity lithium iron phosphate battery.
The preparation method of the reduced graphene oxide-aluminum modified lithium iron phosphate is the same as that in example 1.
Comparative example 1
The present comparative example is different from example 1 only in that unmodified general lithium iron phosphate is used in the preparation of the positive electrode sheet.
Comparative example 2
The comparative example is different from example 1 only in that aluminum powder is not added in the step (a) in the preparation process of the reduced graphene oxide-aluminum modified lithium iron phosphate to reduce graphene oxide.
Comparative example 3
The comparative example is different from example 1 only in that graphene oxide is replaced with graphene in step (a) in the preparation process of the reduced graphene oxide-aluminum modified lithium iron phosphate.
Comparative example 4
The comparative example is different from example 1 only in that step (a) is not included in the preparation process of the reduced graphene oxide-aluminum modified lithium iron phosphate, and the reduced graphene oxide-aluminum solution in step (b) is replaced with an aluminum ion solution of the same concentration.
Comparative example 5
The comparative example is different from example 1 only in that the addition amount of the aluminum powder in the step (a) in the preparation process of the reduced graphene oxide-aluminum modified lithium iron phosphate is 10 times of the mass of the graphene oxide.
Comparative example 6
The present comparative example is different from example 1 only in that aluminum in step (a) is replaced with zinc in an equivalent amount during the preparation of reduced graphene oxide-aluminum modified lithium iron phosphate.
Comparative example 7
The present comparative example is different from example 1 only in that aluminum in step (a) is replaced with magnesium in an equivalent amount during the preparation of reduced graphene oxide-aluminum modified lithium iron phosphate.
Comparative example 8
The comparative example is different from example 1 only in that the high-temperature sintering time in step (c) in the preparation process of the reduced graphene oxide-aluminum modified lithium iron phosphate is 6 hours.
Comparative example 9
The comparative example is different from example 1 only in that the formation step in the preparation process of the reduced graphene oxide-aluminum modified lithium iron phosphate is as follows: the cell was charged with a current of 0.07C for 20min, then left to stand at 38 ℃ for 10h, and then charged to 2.45V with a current of 0.3C.
Comparative example 10
The difference between the comparative example and the example 1 is that the formation step in the preparation process of the reduced graphene oxide-aluminum modified lithium iron phosphate is as follows: the cell was charged with a current of 0.07C for 140min, then left to stand at 38 ℃ for 60h, and then charged to 2.45V with a current of 0.3C.
For the lithium iron phosphate batteries prepared in examples 1 to 5 and comparative example 1 of the present invention, the cycling performance at different current intensities and temperatures was tested within the voltage range of 2.5 to 4.2V, and the results are shown in table 1.
TABLE 1 cycling performance of lithium iron phosphate batteries
As can be seen from the above-mentioned examples 1 to 5, the lithium iron phosphate battery prepared by the method of the present invention has good stability at normal temperature or high temperature, the capacity retention rate after 1000 cycles at 25 ℃ is about 94%, and the capacity retention rate after 5000 cycles is still about 85%, which indicates that the conductivity of the lithium iron phosphate battery positive electrode material using the reduced graphene oxide-aluminum modified lithium iron phosphate positive electrode material is higher, and the long-cycle capacity retention rate of the battery at high temperature can be improved by the added electrolyte solvent system due to the use of the lithium iron phosphate battery positive electrode material in combination with the screened electrolyte. In addition, a reasonable formation process is adopted, so that an SEI film formed on a positive electrode and a negative electrode is ensured to have reasonable thickness, and the specific capacity of the battery is reduced due to reduction and thickening in the subsequent long-cycle process.
In comparative example 1, the conventional lithium iron phosphate is used for battery preparation, and the capacity retention rates of the obtained lithium iron phosphate battery at different charging rates at normal temperature and high temperature are lower than those of examples 1-5. The comparative example 2 is that aluminum powder is not added to reduce graphene oxide, which is equivalent to that graphene oxide is directly added to the lithium iron phosphate material, and the specific capacity and the capacity retention rate of the prepared lithium iron phosphate material are also obviously reduced due to poor self conductivity of the graphene oxide. Compared with the reduced graphene oxide prepared by an aluminum/acid system chemical reaction and a high-temperature two-step reduction in the preparation process of the scheme, the lithium iron phosphate material is improved by directly adding the common graphene and the aluminum, and the improvement of the specific capacity and the specific capacity retention rate of the lithium iron phosphate material is limited due to the fact that the structure of the conventional graphene is not as good as that of the reduced graphene oxide dispersed after the reaction in the scheme on one hand, and the reduced graphene oxide-aluminum complex is prepared in the reduction process in the scheme on the other hand, so that the conductive capacity of aluminum ions and the dispersibility in the material are enhanced, and the conductivity of the anode material is further enhanced.
The comparative example 4 directly adopts an aluminum ion salt solution to modify the lithium iron phosphate cathode material, so that the introduced aluminum ions are difficult to uniformly disperse in the cathode material, and the modification effect is not as good as that of reducing a graphene oxide-aluminum composite or directly modifying graphene. In comparative example 5, excessive aluminum powder is added, which may cause that part of aluminum metal still remains in the positive electrode material in the subsequent sintering process, and may be melted in the sintering process due to the relatively low melting point of aluminum, which affects the sintering efficiency and the structure of the obtained positive electrode material, so that the specific capacity retention rate of the positive electrode material under high rate current is significantly reduced. Comparative example 6 reduction of graphene oxide with zinc and introduction of zinc ions into lithium iron phosphate positive electrode material resulted in positive electrode material with limited improvement in specific capacity and specific capacity retention. The reason may be that zinc has a standard reduction potential higher than that of aluminum and has a reduction capability weaker than that of aluminum, and thus has a poor reduction effect on graphene oxide. Comparative example 7, magnesium is used to reduce graphene oxide, and it is also impossible to modify the lithium iron phosphate positive electrode material well, which may be caused by that the reactivity of magnesium is too high, and the generation speed of nascent hydrogen is too fast, so that the reaction is difficult to control, and the reduction effect is not good. The sintering time is short in comparative example 8, and thus it may result in insufficient high-temperature reduction of graphene oxide, and a portion remains, thus adversely affecting the conductivity of the cathode material. Comparative example 9 employs too short formation charging and stabilization time, so it is difficult to form SEI films of appropriate thickness on the positive and negative electrodes of the battery, and too short stabilization time may not allow the positive electrode of the battery to be infiltrated with sufficient electrolyte, resulting in significant degradation of cycle performance of the battery. Comparative example 10 adopts excessively long formation charging and stabilization time, which reduces production efficiency on the one hand, and on the other hand, if the charging time is excessively long, the gas generated in the electrolyte is too much, which causes battery swelling and increases safety risk, and simultaneously, SEI films with proper thickness are difficult to form on the surfaces of the positive and negative electrodes.
Claims (9)
1. The high-conductivity long-cycle lithium iron phosphate battery is characterized by comprising a positive plate, a negative plate, a diaphragm and electrolyte, wherein the positive plate is a carbon-coated aluminum foil coated with positive slurry comprising reduced graphene oxide-aluminum modified lithium iron phosphate.
2. The high-conductivity long-cycle lithium iron phosphate battery as claimed in claim 1, wherein the positive electrode slurry further comprises a conductive agent and a binder, and the components are calculated according to the following parts by weight: 90-95 parts of reduced graphene oxide-aluminum modified lithium iron phosphate, 1-5 parts of a conductive agent and 1-3 parts of an adhesive.
3. The high-conductivity long-cycle lithium iron phosphate battery according to claim 1, wherein the negative electrode sheet is a copper foil coated with a negative electrode slurry, and the negative electrode slurry comprises a negative electrode material, a conductive agent and a binder; the negative electrode material is one of natural graphite, artificial graphite, hard carbon, soft carbon and lithium metal.
4. The high-conductivity long-cycle lithium iron phosphate battery according to claim 2 or 3, wherein the conductive agent is one of carbon nanotubes, ketjen black, acetylene black and conductive carbon black or a combination thereof, and the binder is one of CMC, polyvinylidene fluoride and polyvinyl alcohol.
5. The high electrical conductivity long cycle lithium iron phosphate cell of claim 1, wherein the electrolyte comprises a lithium salt, an additive, and a solvent; the lithium salt is 1-1.3 mol/L lithium hexafluorophosphate, the solvent is a composition of at least two of ethylene carbonate, propylene carbonate, ethyl acetate and ethyl propionate, and the additive is one of chloroethylene, ethylene sulfate, vinylene carbonate, ethylene carbonate and diethyl carbonate.
6. A method for preparing a high-conductivity long-cycle lithium iron phosphate battery as claimed in any one of claims 1 to 5, comprising the steps of:
(1) preparing a positive plate: preparing reduced graphene oxide-aluminum modified lithium iron phosphate, coating the positive electrode slurry containing the reduced graphene oxide-aluminum modified lithium iron phosphate on the surface of a carbon-coated aluminum foil, and cutting and baking to obtain a positive electrode plate;
(2) preparing a negative plate: mixing a negative electrode material, a binder and a conductive agent to obtain a negative electrode slurry, coating the negative electrode slurry on the surface of copper foil, and cutting and baking to obtain a negative electrode plate;
(3) assembling the battery: assembling a positive pole piece, a negative pole piece, a diaphragm and electrolyte;
(4) formation and capacity grading: the formation steps are as follows: charging the battery for 60-90 min by using a current of 0.05-0.1C, standing the battery for 24-48 h at 35-38 ℃, charging the battery to 3.3-3.65V by using a current of 0.2-0.5C, and then carrying out capacity grading to obtain the high-conductivity long-cycle lithium iron phosphate battery.
7. The method for preparing the high-conductivity long-cycle lithium iron phosphate battery according to claim 6, wherein the battery assembling step comprises rolling, slitting, sheet making, winding, assembling, top-side sealing, drying and electrolyte injection.
8. The method for preparing the high-conductivity long-cycle lithium iron phosphate battery according to claim 6, wherein the method for preparing the reduced graphene oxide-aluminum modified lithium iron phosphate comprises the following steps:
(a) dissolving graphene oxide in water, ultrasonically dispersing for 1-4 hours to prepare a solution with the mass fraction of 0.5-1%, adding an HCl solution with the mass fraction of 30-35%, wherein the volume ratio of the graphene oxide solution to the HCl solution is 3-5: 1, adding aluminum powder with the mass of 4-6 times that of the graphene oxide, and reacting for 30-60 min to prepare a reduced graphene oxide-aluminum solution;
(b) adding an iron source, a phosphorus source and a lithium source into the reduced graphene oxide-aluminum solution, wherein the mass ratio of the total mass of the iron source, the phosphorus source and the lithium source to the mixed solution is (0.5-2): 1, uniformly mixing, heating to 60-80 ℃, and reacting for 2-4 hours;
(c) drying at 120-180 ℃ for 12-18 h, and sintering at 600-900 ℃ for 8-12 h to obtain the reduced graphene oxide-aluminum modified lithium iron phosphate cathode material.
9. The high-conductivity long-cycle lithium iron phosphate battery according to claim 8, wherein the iron source is one of iron powder, ferrous oxalate, ferrous sulfate, and ferrous nitrate; the phosphorus source is one of phosphoric acid and ammonium dihydrogen phosphate; the lithium source is one of lithium carbonate, lithium acetate and lithium hydroxide; the ratio of the amounts of the iron source, the phosphorus source and the lithium source is (0.95-1.05): (0.9-1.1): 0.95-1.05).
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