CN116002648A - High-compaction low-temperature lithium iron phosphate positive electrode material, and preparation method and application thereof - Google Patents
High-compaction low-temperature lithium iron phosphate positive electrode material, and preparation method and application thereof Download PDFInfo
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Abstract
The invention provides a high-compaction low-temperature lithium iron phosphate positive electrode material, and a preparation method and application thereof, wherein the preparation method comprises the following steps: (1) Mixing the precursor solution and citrate in an organic solvent for hydrothermal reaction to obtain a first precursor, mixing the first precursor and a binder for spray pyrolysis and granulation to obtain large-particle lithium iron phosphate; (2) Carrying out hydrothermal reaction on the precursor solution to obtain small-particle lithium iron phosphate; (3) Mixing large-particle lithium iron phosphate and small-particle lithium iron phosphate to obtain a lithium iron phosphate anode material; the step (1) and step (2) precursor solutions each independently include an iron source, a phosphorus source, and a lithium source. According to the invention, citrate and an organic solvent are added in the hydrothermal process to generate nanoscale primary particles, and a binder is added to generate large-particle lithium iron phosphate with high lithium ion mobility and excellent low-temperature performance through spray pyrolysis, and the large-particle lithium iron phosphate is mixed with small-particle lithium iron phosphate generated by the hydrothermal process, so that the low-temperature performance and compaction density of the whole material are improved.
Description
Technical Field
The invention belongs to the field of lithium battery anode materials, and relates to a high-compaction low-temperature lithium iron phosphate anode material, a preparation method and application thereof.
Background
The lithium ion battery is used as a new generation green energy source, has the advantages of high voltage, high energy density, long service life, portability and the like, and plays an important role in the fields of mobile equipment, public transportation, aerospace and the like, wherein the positive electrode material is used as an important component of the lithium ion battery, and influences the research and development of the lithium ion battery.
The lithium iron phosphate is used as the positive electrode material of the lithium ion battery, the theoretical capacity of the lithium iron phosphate is up to 170mAh/g, meanwhile, the source of the lithium iron phosphate raw material is wide, the price is low, the material thermal stability is good, the voltage platform is high, the cycle life is long, the lithium iron phosphate is nontoxic and harmless, and no oxygen is separated out under the abuse condition, so that the safety problem which cannot be solved by lithium cobalt oxide and other existing positive electrode materials can be solved, and the lithium iron phosphate is the first choice of the current power type and energy storage type positive electrode material of the lithium ion battery. However, lithium iron phosphate materials are usually prepared by a hydrothermal method or high-temperature solid-phase sintering, and materials prepared by the existing hydrothermal method have low compaction density, and generally have the maximum of 2.2g/cm 3 And the material prepared by the hydrothermal method is nano-sized particles, so that homogenization is difficult, processability is poor, and the whole performance of the battery cell is influenced. In addition, the application range of the lithium ion battery is wider and wider, the low-temperature performance of the lithium ion battery also needs to be improved, the main mode for improving the low-temperature performance of the lithium iron phosphate in the prior art is to add magnetic substances or reduce the particle size through nano grinding, and the modes generally reduce the compaction density of the material while improving the low-temperature performance of the material, so that the lithium iron phosphate cannot meet the requirement of a high specific energy battery core; if the particle size of the primary particles of lithium iron phosphate is increased to improve compaction, the ionic conductivity of the material is reduced, and the requirement of a low-temperature battery cell cannot be met.
In summary, the existing lithium iron phosphate preparation method cannot improve the compaction density and low-temperature performance of the material at the same time, so that the material cannot have good electrochemical performance at normal temperature and low temperature; therefore, the preparation method of the lithium iron phosphate positive electrode material with compaction and low-temperature performance is provided, and has important significance for research and development of lithium ion batteries.
Disclosure of Invention
Aiming at the problems in the prior art, the invention aims to provide a high-compaction low-temperature lithium iron phosphate positive electrode material, and a preparation method and application thereof. According to the invention, the particle size of primary particles of lithium iron phosphate is regulated by adding citrate and an organic solvent in a hydrothermal process, the binding force among the primary particles and the hardness of the prepared secondary particles are regulated by adding a binder, large-particle lithium iron phosphate with high lithium ion mobility and excellent low-temperature performance is generated after spray pyrolysis, and the large-particle lithium iron phosphate is mixed with small-particle lithium iron phosphate generated by hydrothermal reaction, so that the low-temperature performance and compaction density of the whole lithium iron phosphate positive electrode material are further improved, and the prepared lithium ion battery has higher specific capacity and coulombic efficiency at normal temperature and higher capacity retention rate at low temperature.
In the invention, "high compaction" refers to the fact that when the material is used for preparing pole pieces, the compaction density of the pole pieces is not lower than 2.3g/cm 3 The method comprises the steps of carrying out a first treatment on the surface of the "Normal temperature" means 25 ℃; "Low temperature" means-30 ℃.
In order to achieve the aim of the invention, the invention adopts the following technical scheme:
in a first aspect, the present invention provides a method for preparing a lithium iron phosphate positive electrode material, in particular a method for preparing a high-compaction low-temperature lithium iron phosphate positive electrode material, the method comprising:
(1) Mixing a precursor solution and citrate in an organic solvent for hydrothermal reaction to obtain a first precursor, mixing the first precursor with a binder for spray pyrolysis and granulation to obtain large-particle lithium iron phosphate;
(2) Carrying out hydrothermal reaction on the precursor solution to obtain small-particle lithium iron phosphate;
(3) Mixing the large-particle lithium iron phosphate in the step (1) with the small-particle lithium iron phosphate in the step (2) to obtain a lithium iron phosphate anode material;
the precursor solutions of step (1) and step (2) each independently include an iron source, a phosphorus source, and a lithium source.
Preferably, the preparation method of the precursor solution in the step (1) and the step (2) comprises the following steps:
adding alkali liquor into lithium iron phosphate waste, stirring, performing solid-liquid separation to obtain first filter residue and first filtrate, mixing the first filter residue with the acid liquor, and performing solid-liquid separation to obtain second filter residue and precursor solution.
Preferably, the step of mixing the first filter residue with the acid solution, and after solid-liquid separation, further comprises the step of adding a lithium source to the precursor solution.
Preferably, the citrate of step (1) comprises titanium citrate.
Preferably, the organic solvent of step (1) comprises any one or a combination of at least two of ethylene acrylic acid copolymer, ethylene acetic acid, ethylene copolymer, polyvinyl alcohol derivative, polyethylene glycol and polyethylene glycol derivative.
Preferably, the binder of step (1) comprises styrene butadiene rubber.
Preferably, the weight of the difference between the first filter residue and the second filter residue is recorded as the weight of solid lithium iron phosphate, and the molar ratio of the citrate to the solid lithium iron phosphate in the step (1) is 0.05-2%.
Preferably, the average particle size of the first precursor in the step (1) is 30-60 nm.
Preferably, the average particle size of the large-particle lithium iron phosphate in the step (1) is 1-2 μm.
Preferably, the average particle size of the small-particle lithium iron phosphate in the step (2) is 100-200 nm.
Preferably, after the hydrothermal reaction in the step (2), before the small-particle lithium iron phosphate is obtained, the method further comprises a step of mixing the second precursor obtained by the hydrothermal reaction in the step (2) with a soluble carbon source for spray pyrolysis.
Preferably, the average particle diameter of the second precursor is 100 to 150nm.
Preferably, the first precursor and the binder in step (1) are mixed in such a way that:
and washing and concentrating the first precursor to obtain concentrated slurry, adding a binder into the concentrated slurry, and adjusting the viscosity and the solid content to obtain spray pyrolysis slurry.
Preferably, the mass ratio of the binder to the concentrated slurry is 0.5 to 4%.
Preferably, the solids content of the concentrated slurry is 70 to 85%.
Preferably, the solid content of the spray pyrolysis slurry is 30-50%, and the viscosity is 1000-2000 cP.
Preferably, a soluble carbon source is also added during the mixing of the first precursor and the binder.
Preferably, the temperature of the hydrothermal reaction in step (1) and step (2) is independently 120 to 300 ℃.
Preferably, the hydrothermal reaction in the step (1) is carried out for 2-20 hours.
Preferably, the hydrothermal reaction in the step (2) is carried out for 4-30 hours.
Preferably, the spray pyrolysis temperature in step (1) and step (2) is independently 700-800 ℃.
Preferably, the gas in the atmosphere of the spray pyrolysis in step (1) and step (2) independently comprises nitrogen or a nitrogen-hydrogen mixture.
Preferably, in the step (3), the mass ratio of the large-particle lithium iron phosphate to the small-particle lithium iron phosphate is x (10-x), and x is 1-4.
Preferably, the preparation method comprises the following steps:
(1) Adding alkali liquor into lithium iron phosphate waste, stirring, carrying out solid-liquid separation to obtain first filter residue and first filtrate, mixing the first filter residue with acid liquor, carrying out solid-liquid separation to obtain second filter residue and precursor solution, and supplementing a lithium source into the precursor solution;
(2) Mixing the precursor solution added with the lithium source and citrate in an organic solvent, performing hydrothermal reaction for 2-20 h at 120-300 ℃ to obtain a first precursor with the average particle size of 30-60 nm, washing and concentrating the first precursor to obtain concentrated slurry with the solid content of 70-85%, adding a binder and a soluble carbon source into the concentrated slurry to mix, adjusting the solid content to 30-50%, and the viscosity to 1000-2000 cP to obtain spray cracking slurry, and performing spray cracking and granulation on the spray cracking slurry at the temperature of 700-800 ℃ under nitrogen or nitrogen-hydrogen mixed gas to obtain large-particle lithium iron phosphate with the average particle size of 1-2 mu m;
(3) Carrying out hydrothermal reaction on the precursor solution added with the lithium source at 120-300 ℃ for 4-30 hours to obtain a second precursor with the average particle size of 100-150 nm, mixing the second precursor with a soluble carbon source, and carrying out spray pyrolysis under nitrogen or nitrogen-hydrogen mixed gas at 700-800 ℃ to obtain small-particle lithium iron phosphate with the average particle size of 100-200 nm;
(4) Mixing the large-particle lithium iron phosphate and the small-particle lithium iron phosphate according to the mass ratio of x (10-x), wherein x is 1-4, and obtaining the lithium iron phosphate anode material.
In a second aspect, the invention also provides a lithium iron phosphate positive electrode material, in particular a high-compaction low-temperature lithium iron phosphate positive electrode material, which is prepared by adopting the preparation method according to the first aspect.
In a third aspect, the present invention provides a lithium ion battery, wherein the positive electrode of the lithium ion battery comprises the lithium iron phosphate positive electrode material according to the second aspect.
Compared with the prior art, the invention has the following beneficial effects:
(1) The invention prepares nano-level lithium iron phosphate primary particles by combining citrate and an organic solvent with a hydrothermal method, then carries out spray pyrolysis granulation treatment, and blends the prepared large-particle lithium iron phosphate and small-particle lithium iron phosphate to ensure that the compacted density of the pole piece reaches 2.5g/cm 3 The problems of low compaction density of the recycled material by a hydrothermal method, difficulty in homogenization of the prepared nano-scale particles and poor processability in the prior art are solved.
(2) After the lithium iron phosphate primary particles (first precursor) are prepared by a hydrothermal method, the binder is matched with the primary particles by spray pyrolysis and granulation, the binder is decomposed into carbon which is coated on the surfaces of the lithium iron phosphate primary particles in a short time, the tightness among the primary particles in the lithium iron phosphate is enhanced, and the compression resistance of the formed large-particle lithium iron phosphate is improved in the rolling process on the basis of maintaining excellent low-temperature performance characteristics; the mixed small-particle lithium iron phosphate is the particles of the hydrothermal method, the consistency of primary particles is greatly improved, the consistency of produced materials is greatly improved, the compaction density of the finally prepared material can meet EV high compaction requirements, the low-temperature and power performances of the material are obviously improved, meanwhile, the high-temperature solid-phase roasting is not used, the equipment cost is greatly reduced, the production time is reduced, the efficiency is improved, and the problems that the compaction density of the material prepared by the low-temperature lithium iron phosphate process in the prior art is low, the high specific energy EV battery core requirements cannot be met, and the material can only be applied to PHEV projects with low-temperature requirements are solved.
(3) According to the scheme, the hydrothermal method is used for controlling the production of nanoscale lithium iron phosphate primary particles, micron-sized large-particle lithium iron phosphate is formed through granulation, the low-temperature good characteristic of the particles of the hydrothermal method is maintained, meanwhile, a micron-sized particle material with high internal compactness can be formed by matching with a binder, the compaction density of the material after blending can be greatly improved, the slightly large lithium iron phosphate primary particles prepared by the hydrothermal method are used as small-particle lithium iron phosphate for blending, the overall low-temperature performance of the blended material can be further improved, meanwhile, the crystallinity of the particles of the hydrothermal method is high, the consistency is good, the pressure resistance is better than that of the granulated material by the ferric phosphate process, the use of the blended material under the condition of higher compaction density can be supported, and the problems that in the prior art, the ionic conductivity is reduced after the particle size of the phosphate material is increased and compacted, and the low-temperature performance and the compaction density cannot be simultaneously improved are solved.
Drawings
Fig. 1 is an SEM image of the lithium iron phosphate positive electrode material prepared in example 1 of the present invention.
Fig. 2 is an SEM partial enlarged view of the lithium iron phosphate positive electrode material prepared in example 1 of the present invention.
Fig. 3 is a graph of a first charge and discharge at normal temperature of the lithium iron phosphate positive electrode material prepared in example 1 of the present invention.
Fig. 4 is a graph showing the comparison of the discharge capacities at normal and low temperatures of the lithium iron phosphate positive electrode material prepared in example 1 of the present invention, wherein the abscissa represents the capacity ratio of-30 ℃ discharge capacity to 25 ℃ discharge capacity.
Detailed Description
The technical scheme of the invention is further described by the following specific embodiments. It will be apparent to those skilled in the art that the examples are merely to aid in understanding the invention and are not to be construed as a specific limitation thereof.
The lithium iron phosphate is a lithium ion battery anode material with low price, good thermal stability and long cycle life, but the compaction density of the lithium iron phosphate material prepared by a hydrothermal method in the prior art is low, the requirement of a high specific energy battery core cannot be met, if the particle size of primary particles of the lithium iron phosphate is increased to improve compaction, the ion conductivity of the material is reduced, and the requirement of low-temperature performance of the battery cannot be met.
In this regard, the inventors found that the particle size of primary particles is adjusted by adding citrate and an organic solvent in a hydrothermal process, and then the binding force between the primary particles and the hardness of the prepared secondary particles are adjusted by adding a binder in a spray-pyrolysis manner, so that large-particle lithium iron phosphate with high lithium ion mobility and excellent low-temperature performance is generated, and the low-temperature performance and the compaction density of the whole lithium iron phosphate positive electrode material are further improved by utilizing the cooperation between specific large-particle lithium iron phosphate and small-particle lithium iron phosphate generated by a hydrothermal reaction, thereby completing the invention.
According to one aspect of the present invention, there is provided a method for preparing a lithium iron phosphate positive electrode material, in particular, a method for preparing a high-compaction low-temperature lithium iron phosphate positive electrode material, the method comprising:
(1) Mixing the precursor solution and citrate in an organic solvent for hydrothermal reaction to obtain a first precursor, mixing the first precursor and a binder for spray pyrolysis and granulation to obtain large-particle lithium iron phosphate;
(2) Carrying out hydrothermal reaction on the precursor solution to obtain small-particle lithium iron phosphate;
(3) Mixing the large-particle lithium iron phosphate in the step (1) with the small-particle lithium iron phosphate in the step (2) to obtain a lithium iron phosphate anode material;
the step (1) and step (2) precursor solutions each independently include an iron source, a phosphorus source, and a lithium source.
On the one hand, the precursor solution and the citrate are subjected to hydrothermal reaction in an organic solvent, and lithium iron phosphate primary particles (first precursor) with uniform and stable particle size and smaller size are generated in the hydrothermal process through the crosslinking reaction of the citrate and the organic solvent, and the lithium iron phosphate primary particles are used as precursors of large-particle lithium iron phosphate, so that compared with the lithium iron phosphate prepared by a conventional iron phosphate grinding process, the lithium ion mobility and low-temperature capacity of the prepared large-particle lithium iron phosphate can be improved; meanwhile, the first precursor and the binder are mixed and then subjected to spray pyrolysis, the binder is decomposed into carbon to be coated on the surface of primary particles (first precursor) of the lithium iron phosphate, the tightness degree and the binding force between the primary particles of the lithium iron phosphate are enhanced, the hardness of secondary particles (large-particle lithium iron phosphate) obtained through preparation is enhanced, the compression resistance of the large-particle lithium iron phosphate in the rolling process is improved on the basis that the excellent low-temperature performance is kept, the flexibility of the pole piece can be ensured to meet the processing requirement under the condition of high compaction density, and the compaction density of powder is improved.
And secondly, the invention replaces high-temperature solid phase reaction by spray pyrolysis as a carbon coating process, and prepares lithium iron phosphate secondary particles (large-particle lithium iron phosphate) together with granulation, and the binder is decomposed into carbon coated on the surfaces of primary particles of the lithium iron phosphate in a short time, so that the tightness degree between the primary particles in the lithium iron phosphate is enhanced, and the compression resistance of the formed large-particle lithium iron phosphate in the rolling process is increased on the basis of keeping excellent low-temperature performance characteristics.
On the other hand, the invention also adopts a hydrothermal method to prepare the small-particle lithium iron phosphate with controllable granularity, smaller granularity, high crystallinity and high primary particle consistency, thereby improving the ion migration rate and capacity of the small-particle lithium iron phosphate, then mixing the prepared two large-particle lithium iron phosphate with different granularity and the small-particle lithium iron phosphate, and the whole process does not need high-temperature solid-phase roasting, thereby reducing the cost, improving the efficiency and obviously improving the low-temperature performance and the compaction density of the whole lithium iron phosphate anode material. In the invention, the order of the step (1) and the step (2) is not limited, and the operation of preparing the large-particle lithium iron phosphate in the step (1) can be performed first, or the operation of preparing the small-particle lithium iron phosphate in the step (2) can be performed first, and only the large-particle lithium iron phosphate and the small-particle lithium iron phosphate are prepared, and the preparation order of the large-particle lithium iron phosphate and the small-particle lithium iron phosphate does not influence the performance of the final lithium iron phosphate anode material.
It should be noted that "independently" in the present invention means that the two choices do not interfere with each other; for example, the precursor solutions of step (1) and step (2) each independently include an iron source, a phosphorus source and a lithium source, which means that the precursor solutions of step (1) and step (2) each contain an iron source, a phosphorus source and a lithium source, but the iron sources in the precursor solutions of the two may be the same iron source or different iron sources, and the selection of the two does not affect each other. In addition, the iron source, the phosphorus source and the lithium source in the precursor solution are not particularly limited in the present invention, and only Fe, P and Li ions need to be contained.
In some embodiments, the hydrothermal reactions of step (1) and step (2) are performed in different reaction vessels.
In some embodiments, the method of preparing the precursor solutions of step (1) and step (2) comprises:
adding alkali liquor into lithium iron phosphate waste, stirring, performing solid-liquid separation to obtain first filter residue and first filtrate, mixing the first filter residue with the acid liquor, and performing solid-liquid separation to obtain second filter residue and precursor solution.
In the present invention, the source of the lithium iron phosphate waste is not particularly limited, and it may be obtained by disassembling the positive electrode sheet of the lithium iron phosphate battery, for example.
In the invention, the recycling solution of the lithium iron phosphate waste is preferably used as a precursor solution, and alkali liquor is added into the lithium iron phosphate waste to disable the binder and dissolve and remove impurity aluminum element; then adding acid solution to dissolve and separate the obtained first filter residue, dissolving Fe, P, li and other elements in the lithium iron phosphate in the acid solution, and completely removing insoluble impurities such as conductive agent, copper and the like through solid-liquid separation to obtain a precursor solution.
The recovery and reuse mode of the lithium iron phosphate waste material can extract and dissolve Fe, P and Li elements in the lithium iron phosphate waste material to be close to 100%, and residual aluminum, copper and other ions in the pole piece disassembling process can be removed through a process, so that the low impurity content of the prepared material is ensured, and the consistency of the material can meet the requirements of power products. Meanwhile, the low-temperature performance and the power performance of the material produced by the recycling process are both improved compared with those of the raw material before regeneration, the compaction density is greatly improved, and the consistency of the product is improved. In addition, compared with the hydrometallurgy process flow from the recovery of raw materials to the production of the anode material, the process flow, the equipment and the pollutant discharge are reduced because the processes of precipitation, extraction, crystallization and the like are not carried out by taking elements as units, the recovery efficiency is greatly improved, the produced wastewater can be reused after simple treatment, the solid-phase high-temperature roasting equipment and process are eliminated, the equipment investment cost and the time cost are greatly reduced, the cost advantage is obvious, and the development of the low-carbonization industry of the anode material of the lithium iron phosphate battery is facilitated. The prepared precursor solution is used for generating large-particle lithium iron phosphate in a part later, and is used for generating small-particle lithium iron phosphate in a part later, so that the cost is reduced, a high-temperature solid-phase roasting process is omitted, the production time is shortened, the production efficiency is improved, and the low-temperature performance and the compaction performance of the finally prepared lithium iron phosphate anode material are further improved.
In some embodiments, the lye comprises sodium hydroxide solution.
In some embodiments, the concentration of sodium hydroxide solution is 10-40%, for example, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or the like.
In some embodiments, the stirring time for adding the lye and stirring is 0.5-2 h, for example, 0.5h, 0.8h, 1h, 1.2h, 1.4h, 1.6h, 1.8h, 2h, etc.
In some embodiments, the stirring temperature for stirring the alkali solution is 40-80 ℃, and may be 40 ℃, 45 ℃, 50 ℃, 55 ℃, 60 ℃, 65 ℃, 70 ℃, 75 ℃, 80 ℃ or the like.
In some embodiments, the acid solution comprises a sulfuric acid solution.
In some embodiments, the acid concentration is 1 to 9.2mol/L, and may be, for example, 1mol/L, 2mol/L, 3mol/L, 4mol/L, 5mol/L, 6mol/L, 7mol/L, 8mol/L, 9mol/L, 9.2mol/L, or the like.
In some embodiments, the molar ratio of acid in the acid solution to lithium iron phosphate in the lithium iron phosphate waste is 1 (0.9-0.95), such as may be 1:0.9, 1:0.91, 1:0.92, 1:0.93, 1:0.94, or 1:0.95, etc.
In some embodiments, the first filter residue and the acid solution are mixed, and the solid-liquid separation is performed by a fine filtration, where the fine filtration has a precision of less than 0.1 μm, for example, 0.1 μm, 0.09 μm, 0.08 μm, 0.07 μm, 0.06 μm, or 0.05 μm, etc.
In some embodiments, the first filter residue and the acid solution are mixed, and after solid-liquid separation, the method further comprises the operation of supplementing a lithium source into the precursor solution, wherein the lithium source is supplemented, so that the Li element in the precursor solution is sufficient, and the subsequent preparation of lithium iron phosphate is utilized.
In some embodiments, the molar ratio of the additional lithium source to the first filter residue is (1.5-2.5): 1, which may be, for example, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, or 2.5:1, etc.
In some embodiments, the supplemental lithium source includes any one or a combination of at least two of lithium hydroxide, lithium acetate, and lithium chloride, for example, a combination of lithium hydroxide and lithium acetate, a combination of lithium acetate and lithium chloride, or a combination of lithium hydroxide, lithium acetate, and lithium chloride, and the like.
In some embodiments, the citrate salt of step (1) comprises titanium citrate
In some embodiments, the organic solvent of step (1) includes a solution of any one of ethylene acrylic acid copolymer, ethylene acetic acid, ethylene copolymer, polyvinyl alcohol derivative, polyethylene glycol, and polyethylene glycol derivative or a mixed solution of at least two thereof, and may be, for example, an aqueous solution of polyethylene glycol, a mixed solution of ethylene acrylic acid copolymer and ethylene acetic acid, a mixed solution of polyvinyl alcohol and polyethylene glycol, a mixed solution of ethylene copolymer, polyvinyl alcohol and polyvinyl alcohol derivative, or a mixed solution of ethylene acrylic acid copolymer, ethylene acetic acid, ethylene copolymer, polyvinyl alcohol and polyethylene glycol, or the like.
In some embodiments, the mass of solute in the organic solvent of step (1) is 10-60% of the total mass of the organic solvent, which may be, for example, 10%, 20%, 30%, 40%, 50% or 60%, etc.
The solute of the organic solvent means an active ingredient in the organic solvent, that is, an ingredient capable of undergoing a crosslinking reaction with citrate, and examples thereof include ethylene acrylic acid copolymer, ethylene acetic acid, ethylene copolymer, polyvinyl alcohol derivative, polyethylene glycol derivative, and the like.
According to the invention, through the crosslinking reaction of the citrate and the organic solvent, the first precursor with uniform and stable particle size and smaller size is synthesized in the hydrothermal reaction process to serve as the precursor of the large-particle lithium iron phosphate.
In some embodiments, the binder of step (1) comprises styrene-butadiene rubber as a binder and which does not retain other elements, which is advantageous for improving the properties of the resulting large-particle lithium iron phosphate.
In some embodiments, the weight of the difference between the first filter residue minus the second filter residue is expressed as the weight of solid lithium iron phosphate, and the molar ratio of citrate to solid lithium iron phosphate in step (1) is 0.05-2%, for example, 0.05%, 0.1%, 0.15%, 0.2%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8% or 2%, etc.
The invention adopts the citrate titanium with proper content, is favorable for stabilizing the particle size of the hydrothermal method, and can lead to smaller particles when the content of the citrate is higher, and uneven particles and overlarge partial particles when the content of the citrate is lower.
In some embodiments, the average particle size of the first precursor in the step (1) is 30-60 nm, for example, 30nm, 35nm, 40nm, 45nm, 50nm, 55nm or 60nm, and the lithium iron phosphate primary particles (first precursor) with uniform particle size, which are prepared by a hydrothermal method by jointly adjusting citrate and an organic solvent, are used as precursors, so that compared with the material prepared by the traditional iron phosphate grinding process, the prepared large-particle lithium iron phosphate has higher lithium ion mobility and low-temperature capacity.
In some embodiments, the average particle size of the large particle lithium iron phosphate of step (1) is 1-2 μm, which may be, for example, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm or 2 μm, etc., and the large particle lithium iron phosphate particle size is adjusted to 1-2 μm by binder and spray pyrolysis in combination, which facilitates subsequent complexation with small particle lithium iron phosphate, and synergistically enhances compaction and low temperature properties of the material.
In some embodiments, the small particle lithium iron phosphate of step (2) has an average particle size of 100 to 200nm, which may be, for example, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm, 200nm, or the like.
In some embodiments, after the hydrothermal reaction in the step (2) and before the small-particle lithium iron phosphate is obtained, the method further comprises a step of mixing the second precursor obtained in the hydrothermal reaction in the step (2) with a soluble carbon source for spray pyrolysis, and carbon is coated on the surface of the second precursor which is prepared by spray pyrolysis and has high crystallinity, good consistency and controllable particle size, so that the small-particle lithium iron phosphate with the particle size of 100-200 nm is obtained, and the small-particle lithium iron phosphate and the large-particle lithium iron phosphate are better cooperated to play a role in improving compaction and low-temperature performance.
In some embodiments, the second precursor has an average particle size of 100 to 150nm.
In the invention, the first precursor is lithium iron phosphate primary particles, and the large-particle lithium iron phosphate is lithium iron phosphate secondary particles formed by agglomeration and combination of the first precursor. The second precursor is lithium iron phosphate primary particles, and the small-particle lithium iron phosphate is still lithium iron phosphate primary particles because the small-particle lithium iron phosphate only coats a carbon layer on the surface of the second precursor and does not carry out the processes of agglomeration, combination and the like of the second precursor along with the growth of crystals.
In some embodiments, the first precursor and the binder of step (1) are mixed in such a way that:
washing and concentrating the first precursor to obtain concentrated slurry, adding a binder into the concentrated slurry, and adjusting the viscosity and the solid content to obtain spray pyrolysis slurry.
In some embodiments, the mass ratio of binder to concentrated slurry is 0.5-4%, e.g., may be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, or 4%, etc.
According to the invention, the tightness between the primary particles can be improved by adjusting the content of the binder, when the content of the binder is too large, the secondary particles are too large, when the content of the binder is too small, and the conductivity inside the particles is poor.
In some embodiments, the solids content of the concentrated slurry is 70-85%, such as 70%, 72%, 74%, 76%, 78%, 80%, 82%, 84%, 85%, or the like.
In some embodiments, the spray pyrolysis slurry has a solids content of 30-50%, such as 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, or 50%, and a viscosity of 1000-2000 cP, such as 1000cP, 1100cP, 1200cP, 1300cP, 1400cP, 1500cP, 1600cP, 1700cP, 1800cP, 1900cP, 2000cP, or the like.
According to the invention, the binder is added and the spray cracking slurry is adjusted to carry out spray cracking reaction under a certain viscosity to form large-particle lithium iron phosphate with a certain size, so that the effect of the binder on improving the binding power of primary particles and the hardness of secondary particles is exerted, and the lithium iron phosphate anode material with more excellent compaction and low-temperature performance is prepared.
In some embodiments, a soluble carbon source is also added during the mixing of the first precursor and the binder.
In some embodiments, the soluble carbon source added during the mixing of the first precursor and the binder, and during the mixing of the second precursor and the soluble carbon source, independently, includes any one or a combination of at least two of glucose, sucrose, polyvinyl alcohol, and starch, and may be, for example, a combination of glucose and sucrose, a combination of polyvinyl alcohol and starch, or a combination of glucose, sucrose, polyvinyl alcohol, and starch, and the like.
In some embodiments, the mass ratio of the soluble carbon source and the solid lithium iron phosphate added during the mixing of the first precursor and the binder, and during the mixing of the second precursor and the soluble carbon source, is independently 0.5 to 2.5%, for example, may be 0.5%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, or the like.
In some embodiments, the temperature of the hydrothermal reaction of step (1) and step (2) is independently 120 to 300 ℃, for example, 120 ℃, 140 ℃, 160 ℃, 180 ℃, 200 ℃, 220 ℃, 240 ℃, 260 ℃, 280 ℃, 300 ℃, or the like.
In some embodiments, the time of the hydrothermal reaction of step (1) is 2 to 20 hours, which may be, for example, 2 hours, 4 hours, 6 hours, 8 hours, 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, or the like.
In some embodiments, the time of the hydrothermal reaction of step (2) is 4 to 30 hours, which may be, for example, 4 hours, 8 hours, 10 hours, 15 hours, 18 hours, 20 hours, 22 hours, 25 hours, 30 hours, or the like.
In some embodiments, the temperature of the spray pyrolysis of step (1) and step (2) is independently 700-800 ℃, e.g., 700 ℃, 710 ℃, 720 ℃, 730 ℃, 740 ℃, 750 ℃, 760 ℃, 770 ℃, 780 ℃, 790 ℃, 800 ℃, or the like.
In some embodiments, the gas in the atmosphere of step (1) and step (2) spray pyrolysis independently comprises nitrogen or a nitrogen-hydrogen mixture.
In some embodiments, the mass ratio of the large-particle lithium iron phosphate to the small-particle lithium iron phosphate in step (3) is x (10-x), wherein x is 1-4, such as 1, 1.5, 2, 2.5, 3, 3.5 or 4, and the like, and the synergistic effect can be increased by mixing the large-particle lithium iron phosphate and the small-particle lithium iron phosphate in a proper proportion, so that the compaction and low-temperature performance can be further improved.
In some embodiments, the method of making comprises:
(1) Adding alkali liquor into lithium iron phosphate waste, stirring, performing solid-liquid separation to obtain first filter residue and first filtrate, mixing the first filter residue with acid liquor, performing solid-liquid separation to obtain second filter residue and precursor solution, and adding a lithium source into the precursor solution;
(2) Mixing the precursor solution added with the lithium source and citrate in an organic solvent, performing hydrothermal reaction for 2-20 h at 120-300 ℃ to obtain a first precursor with the average particle size of 30-60 nm, washing and concentrating the first precursor to obtain concentrated slurry with the solid content of 70-85%, adding a binder and a soluble carbon source into the concentrated slurry for mixing, adjusting the solid content to 30-50% and the viscosity to 1000-2000 cP to obtain spray cracking slurry, and performing spray cracking and granulation on the spray cracking slurry at 700-800 ℃ under nitrogen or nitrogen-hydrogen mixed gas to obtain large-particle lithium iron phosphate with the average particle size of 1-2 mu m;
(3) Carrying out hydrothermal reaction on the precursor solution after adding the lithium source at 120-300 ℃ for 4-30 hours to obtain a second precursor with the average particle size of 100-150 nm, mixing the second precursor with a soluble carbon source, and carrying out spray pyrolysis under nitrogen or nitrogen-hydrogen mixed gas at 700-800 ℃ to obtain small-particle lithium iron phosphate with the average particle size of 100-200 nm;
(4) Mixing large-particle lithium iron phosphate and small-particle lithium iron phosphate according to the mass ratio of x (10-x), wherein x is 1-4, and obtaining the lithium iron phosphate anode material.
According to another aspect of the invention, a lithium iron phosphate cathode material is provided, and the lithium iron phosphate cathode material is prepared by the preparation method.
According to another aspect of the present invention, there is provided a lithium ion battery, wherein the positive electrode of the lithium ion battery comprises the lithium iron phosphate positive electrode material.
Example 1
The embodiment provides a preparation method of a lithium iron phosphate positive electrode material, which comprises the following steps:
(1) Taking out the positive electrode plate of the scrapped battery cell, separating to obtain powdery lithium iron phosphate waste, adding a sodium hydroxide solution with the mass concentration of 40% into the lithium iron phosphate waste, heating to 70 ℃, stirring, performing solid-liquid separation after 1h to obtain first filter residues and first filtrate, weighing the first filter residues, adding a dilute sulfuric acid solution with the concentration of 2mol/L, stirring, performing solid-liquid separation after stirring until the first filter residues are not dissolved any more, separating to obtain solid which is second filter residues, subtracting the difference weight of the second filter residues from the first filter residues to obtain solid lithium iron phosphate, pumping the separated liquid into a precision filter, and filtering to obtain a precursor solution;
(2) Adding the precursor solution obtained in the step (1) into a reaction kettle containing polyethylene glycol aqueous solution, wherein the mass of polyethylene glycol in the polyethylene glycol aqueous solution is 10% of the total mass of the polyethylene glycol aqueous solution, then adding lithium hydroxide and titanium citrate, mixing and stirring, performing hydrothermal reaction for 4 hours at 160 ℃, controlling the particle size of particles to be 30-50 nm, and obtaining a first precursor, wherein the molar ratio of the lithium hydroxide to the solid lithium iron phosphate is 2:1, and the molar ratio of the titanium citrate to the solid lithium iron phosphate is 0.15%; washing a first precursor by adopting a polyethylene glycol solution, adding the washed solution into a thickener to increase the solid content of the solution by 80%, obtaining concentrated slurry, adding Styrene Butadiene Rubber (SBR) accounting for 1% of the mass of the concentrated slurry and glucose powder accounting for 1.2% of the mass of solid lithium iron phosphate, adding water to adjust the solid content to 40%, the viscosity to 1000cP, performing spray pyrolysis in a nitrogen atmosphere at 700 ℃, and granulating the spray pyrolysis product in a fusion machine to obtain large-particle lithium iron phosphate with the average particle diameter of 1.5 mu m;
(3) Adding the precursor solution obtained in the step (1) into another reaction kettle, adding lithium hydroxide, mixing and stirring uniformly, and performing hydrothermal reaction at 210 ℃ for 6 hours, wherein the average particle size of particles is controlled to be 100-150 nm, so as to obtain a second precursor; washing after the reaction is finished, adding glucose powder with the mass of 0.8% of solid lithium iron phosphate, and performing spray pyrolysis at 750 ℃ in nitrogen atmosphere to obtain small-particle lithium iron phosphate with the average particle size of 100-200 nm;
(4) Mixing large-particle lithium iron phosphate and small-particle lithium iron phosphate according to a mass ratio of 2:8 to obtain the lithium iron phosphate anode material.
The SEM images of the lithium iron phosphate positive electrode material prepared in the embodiment are shown in fig. 1 and 2, fig. 2 is an enlarged view of the circled part in fig. 1, and as can be seen from fig. 1 and 2, the material is obviously formed by mixing two kinds of particles with particle sizes, and the sizes of the small particles of lithium iron phosphate are relatively uniform and consistent, and no obvious fine powder exists; from the enlarged view of the large particle lithium iron phosphate in fig. 2, it can be seen that the large particle lithium iron phosphate is composed of a plurality of small nano-sized particles closely combined, and the interfacial clear carbon coating is clearly visible.
The powder compaction density of the lithium iron phosphate positive electrode material prepared in the embodiment is 2.41g/cm 3 The usable compaction density of the pole piece is 2.58g/cm when the pole piece is prepared 3 。
Example 2
The embodiment provides a preparation method of a lithium iron phosphate positive electrode material, which comprises the following steps:
(1) Taking out the positive plate of the scrapped battery cell, separating to obtain powdery lithium iron phosphate waste, adding 30% sodium hydroxide solution into the lithium iron phosphate waste, heating to 80 ℃ and stirring, carrying out solid-liquid separation after 1h to obtain first filter residue and first filtrate, weighing the first filter residue, adding 2mol/L dilute sulfuric acid solution, stirring, carrying out solid-liquid separation after stirring until the first filter residue is not dissolved, separating to obtain solid which is second filter residue, subtracting the weight of the difference value of the second filter residue from the first filter residue to obtain solid lithium iron phosphate, pumping the separated liquid into a precision filter, and filtering to obtain a precursor solution;
(2) Adding the precursor solution obtained in the step (1) into a reaction kettle containing a polyvinyl alcohol aqueous solution, wherein the mass of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 20% of the total mass of the polyvinyl alcohol aqueous solution, then adding lithium hydroxide and titanium citrate, mixing and stirring, performing hydrothermal reaction for 3 hours at 200 ℃, controlling the average particle size of particles to be 30-50 nm, and obtaining a first precursor, wherein the molar ratio of the lithium hydroxide to the solid lithium iron phosphate is 2:1, and the molar ratio of the titanium citrate to the solid lithium iron phosphate is 0.05%; washing a first precursor by adopting a polyvinyl alcohol solution, adding the washed solution into a thickener to increase the solid content of the solution by 70%, obtaining concentrated slurry, adding SBR (styrene butadiene rubber) with the mass of 0.5% and glucose powder with the mass of 2.5% of solid lithium iron phosphate with the mass of the concentrated slurry, adding water to adjust the solid content to 50%, the viscosity to 2000cP, carrying out spray pyrolysis in a nitrogen atmosphere at 750 ℃, and granulating the spray pyrolysis product in a fusion machine to obtain large-particle lithium iron phosphate with the average particle diameter of 1.2 mu m;
(3) Adding the precursor solution obtained in the step (1) into another reaction kettle, then adding lithium hydroxide, mixing and stirring uniformly, performing hydrothermal reaction for 4 hours at 250 ℃, controlling the average particle size of particles to be 100-150 nm, washing after the reaction is finished, adding glucose powder with the mass of 1% solid lithium iron phosphate, and performing spray pyrolysis at 800 ℃ in nitrogen atmosphere to obtain small-particle lithium iron phosphate with the average particle size of 150-200 nm;
(4) Mixing large-particle lithium iron phosphate and small-particle lithium iron phosphate according to a mass ratio of 1:9 to obtain the lithium iron phosphate anode material.
Example 3
The embodiment provides a preparation method of a lithium iron phosphate positive electrode material, which comprises the following steps:
(1) Taking out the positive electrode plate of the scrapped battery cell, separating to obtain powdery lithium iron phosphate waste, adding a sodium hydroxide solution with the mass concentration of 20% into the lithium iron phosphate waste, heating to 80 ℃, stirring, performing solid-liquid separation after 2 hours to obtain first filter residues and first filtrate, weighing the first filter residues, adding a dilute sulfuric acid solution with the concentration of 3mol/L, stirring, performing solid-liquid separation after stirring until the first filter residues are not dissolved any more, separating to obtain solid which is second filter residues, subtracting the difference weight of the second filter residues from the first filter residues to obtain solid lithium iron phosphate, pumping the separated liquid into a precision filter, and filtering to obtain a precursor solution;
(2) Adding the precursor solution obtained in the step (1) into a reaction kettle containing a mixed solution of ethylene acrylic acid copolymer and ethylene acetic acid in a mass ratio of 1:1, wherein the total mass of the ethylene acrylic acid copolymer and the ethylene acetic acid in the mixed solution of the ethylene acrylic acid copolymer and the ethylene acetic acid is 30% of the total mass of the mixed solution of the ethylene acrylic acid copolymer and the ethylene acetic acid, adding lithium hydroxide and titanium citrate, mixing and stirring, carrying out hydrothermal reaction at 140 ℃ for 10 hours, controlling the average particle size of particles to be 30-50 nm, and obtaining a first precursor, wherein the molar ratio of the lithium hydroxide to the solid lithium iron phosphate is 2:1, and the molar ratio of the titanium citrate to the solid lithium iron phosphate is 2%; washing a first precursor by adopting a polyvinyl alcohol solution, adding the washed solution into a thickener to raise the solid content of the solution by 85%, obtaining concentrated slurry, adding SBR (styrene butadiene rubber) accounting for 4% of the mass of the concentrated slurry and glucose powder accounting for 0.5% of the mass of solid lithium iron phosphate, adding water to adjust the solid content to 40%, adjusting the viscosity to 2000cP, carrying out spray pyrolysis at 700 ℃ in nitrogen atmosphere, and granulating the spray pyrolysis product in a fusion machine to obtain large-particle lithium iron phosphate with the average particle diameter of 2 mu m;
(3) Adding the precursor solution obtained in the step (1) into another reaction kettle, then adding lithium hydroxide, mixing and stirring uniformly, performing hydrothermal reaction for 5 hours at 200 ℃, controlling the average particle size of particles to be 100-150 nm, washing after the reaction is finished, adding glucose powder with the mass of 1.5% of solid lithium iron phosphate, and performing spray pyrolysis at 700 ℃ in nitrogen atmosphere to obtain small-particle lithium iron phosphate with the average particle size of 100-200 nm;
(4) Mixing large-particle lithium iron phosphate and small-particle lithium iron phosphate according to a mass ratio of 4:6 to obtain the lithium iron phosphate anode material.
Example 4
The procedure of example 1 was repeated except that the amount of titanium citrate added in step (2) was changed so that the molar ratio of titanium citrate to solid lithium iron phosphate was 0.03%.
Example 5
The procedure of example 1 was repeated except that the amount of titanium citrate added in step (2) was changed so that the molar ratio of titanium citrate to solid lithium iron phosphate was 2.5%.
Example 6
The procedure of example 1 was repeated except that the titanium citrate in step (2) was replaced with sodium citrate.
Example 7
The procedure of example 1 was repeated except that the amount of SBR added in step (2) was changed so that the mass of SBR was 0.3% of the mass of the concentrated slurry.
Example 8
The procedure of example 1 was repeated except that the amount of SBR added in step (2) was changed so that the mass of SBR was 4.5% of the mass of the concentrated slurry.
Example 9
The procedure of example 1 was repeated except that the SBR in step (2) was replaced with polyvinylidene fluoride.
Example 10
The procedure of example 1 was followed except that water was added in step (2) to adjust the solid content to 60% and the viscosity to 3000 cP.
Example 11
The procedure of example 1 was repeated except that water was added in the step (2) to adjust the solid content to 20% and the viscosity to 500 cP.
Example 12
The procedure of example 1 was repeated except that in step (3), titanium citrate was added to obtain small-particle lithium iron phosphate having an average particle diameter of 30 to 50 nm.
Example 13
The procedure of example 1 was repeated except that the hydrothermal temperature was changed to 200℃for 32 hours in step (3) to obtain small-particle lithium iron phosphate having an average particle diameter of 400 to 500 nm.
Comparative example 1
The procedure of example 1 was repeated except that titanium citrate was not added in the step (2).
Comparative example 2
The procedure of example 1 was repeated except that SBR was not added in step (2).
Comparative example 3
The procedure of example 1 was followed except that the operations of steps (3) and (4) were not performed, and large-particle lithium iron phosphate was directly used as the final lithium iron phosphate cathode material.
Comparative example 4
The procedure of example 1 was followed except that the operations of steps (2) and (4) were not performed, and small-particle lithium iron phosphate was directly used as the final lithium iron phosphate cathode material.
1. Preparation of button cell
The positive electrode material prepared in the example and the comparative example is mixed with the conductive active material SP and the binder PVDF according to the proportion of 90:5:5 to form slurry, and then the slurry is uniformly coated on an aluminum foil to prepare a positive electrode sheetThe negative electrode is made of high-purity lithium sheet, the diaphragm is Celgard 2400, and the electrolyte is LiPF with the concentration of 1mol/L 6 Dissolved in a mixed solvent of EC and DMC (volume ratio is 1:1), and assembled into CR2032 type button cell in a vacuum glove box, and then subjected to electrochemical test.
2. Preparation of 5Ah soft-package battery cell
Mixing the lithium iron phosphate anode material with a conductive active material SP and a binder PVDF according to the proportion of 97:1:2 to form slurry, uniformly coating the slurry on an aluminum foil to prepare an anode plate, selecting artificial graphite as a cathode, assembling an electrolyte TINCTC 6 into a soft-package battery in a vacuum glove box, and then carrying out electrochemical test.
3. Performance testing
(1) Density of compacted powder
The lithium iron phosphate cathode materials prepared in the examples and the comparative examples were placed in a powder compaction density instrument and tested to obtain compacted densities, and the test results are shown in table 1.
(2) Pole piece limit compaction density test
And (3) performing compaction test on the positive plate of the soft-packaged battery core, wherein the experimental data of maximum compaction without breaking is limit compaction.
(3) Charge and discharge test of button cell
The prepared button cell was charged and discharged at a rate of 0.1C at a voltage range of 2 to 3.75V and a temperature of 25C to obtain a specific capacity of 0.1C discharge and coulombic efficiency of the cell, and the results are shown in table 1.
(4) Capacity retention test of 5Ah soft package cell
The prepared 5Ah soft-package battery cell is subjected to charge and discharge tests at-30 ℃ and 25 ℃ in a voltage range of 2-3.65V at a 1C multiplying power, the discharge capacity of the battery cell at different temperatures is recorded, the discharge capacity at-30 ℃ is divided by the discharge capacity at 25 ℃ to obtain the capacity percentage of the battery cell at low temperature, and the results are shown in Table 1.
TABLE 1
As can be seen from examples 1-13, in the invention, the particle size of primary particles of lithium iron phosphate is adjusted by adding citrate and organic solvent in the hydrothermal process, and the binding agent is added to improve the binding force between the primary particles and the hardness of the prepared secondary particles, large-particle lithium iron phosphate with high lithium ion mobility and excellent low-temperature performance is generated after spray pyrolysis, and the large-particle lithium iron phosphate is mixed with small-particle lithium iron phosphate generated by the hydrothermal reaction, so that the overall low-temperature performance and compaction density of the lithium iron phosphate positive electrode material are further improved, and the compaction density of the material in a pole piece is 2.15g/cm 3 Above, even can reach 2.5g/cm 3 Meanwhile, fig. 3 and fig. 4 are a first charge-discharge curve and a low-temperature capacity retention curve of the material prepared in example 1 applied to a lithium ion battery respectively, and it can be seen that the capacity of the material in the invention is greater than 65% at the low temperature of-30 ℃, and the material is greatly improved compared with the existing power type lithium iron phosphate material.
As can be seen from comparison of the embodiment 1 and the embodiments 4-5, by adopting a mixture of citrate with proper content and an organic solvent, 30-50 nm lithium iron phosphate primary particles (first precursor) with uniform and stable particle size can be synthesized in hydrothermal reaction to serve as precursors of large-particle lithium iron phosphate, so that compaction and low-temperature performance of materials are improved; in the embodiment 4, the content of the titanium citrate is smaller, so that the consistency of primary particles of a hydrothermal method is influenced, the particles are larger, and the average particle size of the generated first precursor is 40-100 nm; in the embodiment 5, the content of the titanium citrate is more, which can influence the granularity of primary particles of the hydrothermal method to be lower; thus, the lithium iron phosphate positive electrode material of example 1 has the best overall performance.
As can be seen from comparison of examples 1 and examples 7-8, the compaction and low temperature properties of the material can be further improved by adopting a proper amount of binder to prepare large-particle lithium iron phosphate; the lower content of SBR in example 7 can affect the smaller size of spray cracking particles, the lower conductivity of the interior of secondary particles, and the higher content of binder in example 8 can result in larger secondary particle size; thus, the overall properties of the material of example 1 are more excellent.
By way of example 1 and examples 10-11, the solids content and viscosity of the spray pyrolysis slurry of the present invention affect the electrochemical performance of the final lithium iron phosphate positive electrode material; the slurry in example 10 has higher solid content and viscosity, which affects the granularity of the secondary particles, and the particle size of the large-particle lithium iron phosphate increases; the lower solids content and viscosity of the spray pyrolysis slurry of example 11 resulted in a smaller secondary particle size.
From a comparison of example 1 with examples 12-13, it is seen that the particle size of the small particle lithium iron phosphate affects the compaction performance of the lithium iron phosphate positive electrode material; in example 12, the compaction was affected by adding titanium citrate to reduce the particle size of the small particle lithium iron phosphate; the larger particle size of the small-particle lithium iron phosphate generated in example 13 affects the specific capacity and low-temperature performance of the material; therefore, the lithium iron phosphate positive electrode material prepared in example 1 has more excellent comprehensive properties.
As can be seen from the comparison of the example 1 and the comparative examples 1-2, in the process of preparing large-particle lithium iron phosphate, citrate and a binder are not indispensable, the titanium citrate is not added in the comparative example 1, so that the growth of particles is out of control, and the SBR is not added in the comparative example 2, so that the formation of secondary particles is influenced; thus, the performance of comparative examples 1-2 is significantly reduced compared to example 1.
As can be seen from the comparison of the example 1 and the comparative examples 3 to 4, the blending of the large and small particle lithium iron phosphate prepared by the hydrothermal and spray pyrolysis in the invention can effectively improve the compaction and low temperature properties of the material, and the comparative examples 3 and 4 only use lithium iron phosphate with one particle size, which cannot exert the synergistic effect among particles with a specific size, and the prepared material has significantly poorer comprehensive properties than the example 1.
The foregoing is merely illustrative of the present invention, and the present invention is not limited thereto, and it should be apparent to those skilled in the art that any changes or substitutions that fall within the technical scope of the present invention disclosed herein are within the scope of the present invention.
Claims (10)
1. A method for preparing a lithium iron phosphate positive electrode material, which is characterized by comprising the following steps:
(1) Mixing a precursor solution and citrate in an organic solvent for hydrothermal reaction to obtain a first precursor, mixing the first precursor with a binder for spray pyrolysis and granulation to obtain large-particle lithium iron phosphate;
(2) Carrying out hydrothermal reaction on the precursor solution to obtain small-particle lithium iron phosphate;
(3) Mixing the large-particle lithium iron phosphate in the step (1) with the small-particle lithium iron phosphate in the step (2) to obtain a lithium iron phosphate anode material;
the precursor solutions of step (1) and step (2) each independently include an iron source, a phosphorus source, and a lithium source.
2. The method of claim 1, wherein the precursor solutions of step (1) and step (2) are prepared by a method comprising:
adding alkali liquor into lithium iron phosphate waste, stirring, performing solid-liquid separation to obtain first filter residues and first filtrate, mixing the first filter residues with the acid liquor, and performing solid-liquid separation to obtain second filter residues and precursor solution;
preferably, the step of mixing the first filter residue with the acid solution, and after solid-liquid separation, further comprises the step of adding a lithium source to the precursor solution.
3. The method of preparation according to claim 1 or 2, wherein the citrate salt of step (1) comprises titanium citrate;
preferably, the organic solvent of the step (1) comprises a solution of any one of ethylene acrylic acid copolymer, ethylene acetic acid, ethylene copolymer, polyvinyl alcohol derivative, polyethylene glycol and polyethylene glycol derivative or a mixed solution of at least two of them;
Preferably, the binder of step (1) comprises styrene-butadiene rubber;
preferably, the weight of the difference between the first filter residue and the second filter residue is recorded as the weight of solid lithium iron phosphate, and the molar ratio of the citrate to the solid lithium iron phosphate in the step (1) is 0.05-2%.
4. A method according to any one of claims 1 to 3, wherein the first precursor in step (1) has an average particle diameter of 30 to 60nm;
preferably, the average particle size of the large-particle lithium iron phosphate in the step (1) is 1-2 mu m;
preferably, the average particle size of the small-particle lithium iron phosphate in the step (2) is 100-200 nm;
preferably, after the hydrothermal reaction in the step (2) and before the small-particle lithium iron phosphate is obtained, the method further comprises the step of mixing the second precursor obtained by the hydrothermal reaction in the step (2) with a soluble carbon source for spray pyrolysis;
preferably, the average particle diameter of the second precursor is 100 to 150nm.
5. The method of any one of claims 1-4, wherein the first precursor and the binder of step (1) are mixed in a manner that:
washing and concentrating the first precursor to obtain concentrated slurry, adding a binder into the concentrated slurry, and adjusting the viscosity and the solid content to obtain spray cracking slurry;
Preferably, the mass ratio of the binder to the concentrated slurry is 0.5-4%;
preferably, the solid content of the concentrated slurry is 70-85%;
preferably, the solid content of the spray pyrolysis slurry is 30-50%, and the viscosity is 1000-2000 cP;
preferably, a soluble carbon source is also added during the mixing of the first precursor and the binder.
6. The process of any one of claims 1-5, wherein the temperature of the hydrothermal reaction of step (1) and step (2) is independently 120-300 ℃;
preferably, the hydrothermal reaction in the step (1) is carried out for 2-20 hours;
preferably, the hydrothermal reaction in the step (2) is carried out for 4-30 hours;
preferably, the temperature of the spray pyrolysis in step (1) and step (2) is independently 700-800 ℃;
preferably, the gas in the atmosphere of the spray pyrolysis in step (1) and step (2) independently comprises nitrogen or a nitrogen-hydrogen mixture.
7. The method according to any one of claims 1 to 6, wherein the mass ratio of the large-particle lithium iron phosphate to the small-particle lithium iron phosphate in step (3) is x (10-x), and x is 1 to 4.
8. The method of any one of claims 1-7, wherein the method of preparation comprises:
(1) Adding alkali liquor into lithium iron phosphate waste, stirring, carrying out solid-liquid separation to obtain first filter residue and first filtrate, mixing the first filter residue with acid liquor, carrying out solid-liquid separation to obtain second filter residue and precursor solution, and supplementing a lithium source into the precursor solution;
(2) Mixing the precursor solution added with the lithium source and citrate in an organic solvent, performing hydrothermal reaction for 2-20 h at 120-300 ℃ to obtain a first precursor with the average particle size of 30-60 nm, washing and concentrating the first precursor to obtain concentrated slurry with the solid content of 70-85%, adding a binder and a soluble carbon source into the concentrated slurry to mix, adjusting the solid content to 30-50%, and the viscosity to 1000-2000 cP to obtain spray cracking slurry, and performing spray cracking and granulation on the spray cracking slurry at the temperature of 700-800 ℃ under nitrogen or nitrogen-hydrogen mixed gas to obtain large-particle lithium iron phosphate with the average particle size of 1-2 mu m;
(3) Carrying out hydrothermal reaction on the precursor solution added with the lithium source at 120-300 ℃ for 4-30 hours to obtain a second precursor with the average particle size of 100-150 nm, mixing the second precursor with a soluble carbon source, and carrying out spray pyrolysis under nitrogen or nitrogen-hydrogen mixed gas at 700-800 ℃ to obtain small-particle lithium iron phosphate with the average particle size of 100-200 nm;
(4) Mixing the large-particle lithium iron phosphate and the small-particle lithium iron phosphate according to the mass ratio of x (10-x), wherein x is 1-4, and obtaining the lithium iron phosphate anode material.
9. A lithium iron phosphate positive electrode material, characterized in that the lithium iron phosphate positive electrode material is prepared by the preparation method according to any one of claims 1 to 8.
10. A lithium ion battery, characterized in that the lithium ion battery positive electrode comprises the lithium iron phosphate positive electrode material according to claim 9.
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