CN113716542A - High-capacity high-compaction-density high-iron-phosphorus-ratio nano lithium iron phosphate and preparation method thereof - Google Patents
High-capacity high-compaction-density high-iron-phosphorus-ratio nano lithium iron phosphate and preparation method thereof Download PDFInfo
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Abstract
The invention provides a high-capacity high-compaction-density high-iron-phosphorus-ratio nano lithium iron phosphate and a preparation method thereof, wherein the method comprises the following steps: mixing a phosphorus salt solution and an iron salt solution for oxidation reaction to obtain an oxidation slurry; carrying out first grinding on the oxidation slurry to obtain fine-grain-size oxidation slurry with the D99 being less than or equal to 1 mu m and the D50 being less than or equal to 300 nm; making the thinAfter the particle size slurry is subjected to first washing, adding pure water and phosphoric acid to perform conversion reaction to obtain conversion slurry; adjusting the pH value of the conversion slurry to 6-8, and then carrying out second grinding to obtain a fine particle size conversion slurry with the D99 being less than or equal to 1 mu m and the D50 being less than or equal to 300 nm; then, carrying out second washing, drying and calcining to obtain anhydrous iron phosphate; mixing the anhydrous iron phosphate with lithium carbonate, glucose and pure water, and adding MoO3Graphite and carbon nano tubes are sequentially ground, dried, sintered and crushed in an inert atmosphere to obtain the lithium iron phosphate material, and the lithium iron phosphate material has the advantages of high capacity, high compaction density and high iron-phosphorus ratio.
Description
Technical Field
The invention relates to the technical field of lithium iron phosphate preparation, in particular to high-capacity high-compaction-density high-iron-phosphorus-ratio nano lithium iron phosphate and a preparation method thereof.
Background
Lithium iron phosphate is one of the most fiery anode materials in the lithium battery industry at present, and has an orthogonal olivine structure, and P and O are combined by a covalent bond, so that the structure of the lithium iron phosphate is extremely stable. Therefore, the lithium iron phosphate material has good cycle performance, and the cycle frequency can reach more than 2000 times; meanwhile, the thermal stability is good, and the safety performance is excellent. In addition, the lithium iron phosphate material does not contain precious metal elements, and iron and phosphorus are conventional industrial raw materials, so that the lithium iron phosphate material is low in price, rich in reserves and low in production cost. And the material does not contain any heavy metal elements harmful to human bodies, is green, environment-friendly and pollution-free, and is a real green new energy material.
The theoretical gram capacity of the lithium iron phosphate is 170mAh/g, the voltage platform is about 3.2V, and the real density is 3.6g/cm3. Through years of optimization of technicians in the industry, the gram capacity of the material can reach more than 150mAh/g, and the compaction density can reach 2.4g/cm3The system energy density of the lithium iron phosphate battery can reach 160Wh/kg at most. However, there is still a small gap in energy density compared to the system energy density of the ternary material above 180 Wh/kg. In addition, no matter gram capacity or compaction density, the lithium iron phosphate material has a certain lifting space. Therefore, how to develop a high-capacity high-compaction-density high-iron-phosphorus-ratio nano lithium iron phosphate and a preparation method thereof become a technical problem to be solved urgently.
Disclosure of Invention
Aiming at the technical problems in the prior art, the invention provides nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio and a preparation method thereof, and the prepared nano lithium iron phosphate has the advantages of high capacity, high compaction density and high iron-phosphorus ratio.
The invention adopts the following technical scheme:
the invention provides a preparation method of nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio, which comprises the following steps:
mixing a phosphorus salt solution and an iron salt solution for oxidation reaction to obtain an oxidation slurry;
carrying out first grinding on the oxidized slurry to obtain fine-grain-size oxidized slurry, wherein the fine-grain-size oxidized slurry meets the following requirements: d99 is less than or equal to 1 mu m, and D50 is less than or equal to 300 nm;
after the fine-particle size slurry is subjected to first washing, adding pure water and phosphoric acid to perform conversion reaction to obtain conversion slurry;
adjusting the pH value of the conversion slurry to 6-8, and then carrying out second grinding to obtain conversion slurry with a fine particle size, wherein the conversion slurry with the fine particle size meets the following requirements: d99 is less than or equal to 1 mu m, and D50 is less than or equal to 300 nm;
carrying out second washing on the fine-particle size conversion slurry, drying and calcining to obtain anhydrous iron phosphate;
mixing the anhydrous iron phosphate with lithium carbonate, glucose and pure water, and then adding MoO3And uniformly mixing graphite and the carbon nano tube to obtain a mixture, and then sequentially grinding, drying, sintering and crushing the mixture in an inert atmosphere to obtain the lithium iron phosphate material.
In the oxidation reaction, the mass ratio of the phosphorus element in the phosphorus salt solution to the iron element in the iron salt solution is (1-1.05): 1.
Further, the first washing includes: and filtering the slurry with the fine particle size and washing with pure water, wherein the washing end point is that the conductivity of washing water is less than or equal to 1 mS/cm.
Further, in the conversion reaction, pure water and phosphoric acid are added to control the pH value of the fine particle size slurry to be 1.0-2.2, and the solid content is 10-20 wt%.
Further, the temperature of the conversion reaction is 90-100 ℃, and the time of the conversion reaction is 1-4 hours.
Further, a pH regulator is adopted in the pH regulation, and the pH regulator is at least one of ammonia water, ammonium carbonate and ammonium bicarbonate.
Further, the second washing includes: and filtering the fine particle size conversion slurry, and washing with pure water, wherein the washing end point is that the conductivity of washing water is less than or equal to 200 mu S/cm.
Further, the dry calcination comprises: drying at 120-180 ℃ to remove free water, and calcining at 550-650 ℃ for 2-4 h to remove crystal water.
Further, the anhydrous iron phosphate is mixed with lithium carbonate, glucose and pure water, and then MoO is added3And uniformly mixing the graphite and the carbon nano tube to obtain a mixture, which specifically comprises the following steps:
the anhydrous iron phosphate is mixed with lithium carbonate, glucose and pure water, wherein the mass ratio of the anhydrous iron phosphate to the lithium carbonate is 1: (0.5 to 0.55); the mass of the glucose is 7-8.5 wt% of that of the anhydrous ferric phosphate; the adding amount of the pure water is 40-45 wt% of solid content;
post-addition of MoO3Mixing graphite and carbon nano tube uniformly to obtain a mixture, wherein the MoO3The mass of the medium Mo element is 0.1-0.2 wt% of the theoretical mass of the lithium iron phosphate, the mass of the graphite is 0.1-0.2 wt% of the theoretical mass of the lithium iron phosphate, and the mass of the carbon nano tube is 0.1-0.2 wt% of the theoretical weight of the lithium iron phosphate.
The invention also provides the nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio prepared by the method.
Compared with the prior art, the invention has the beneficial effects that:
the invention provides a preparation method of high-capacity high-compaction-density high-iron-phosphorus-ratio nano lithium iron phosphate, which comprises the following steps of (1) controlling fine-particle-size oxidation slurry through first grinding to meet the following requirements: d99 is less than or equal to 1 mu m, and D50 is less than or equal to 300 nm; the generated amorphous iron phosphate can be fully dispersed, the agglomeration is removed, and impurity ions wrapped in the agglomerated particles are released, so that the next step of water washing is facilitated, and the impurity ions are removed as far as possible; meanwhile, the ground particles become uniform and fine, which is beneficial to the subsequent conversion reaction and improves the conversion rate and the conversion rate. (2) Firstly adjusting the pH value to 6-8, and then carrying out second grinding to control the fine particle size conversion slurry to meet the following requirements: d99 is less than or equal to 1 mu m, and D50 is less than or equal to 300 nm; the phosphoric acid can be converted into ammonium phosphate by adjusting the pH to 6-8, which is beneficial to subsequent waterAnd (5) washing to remove impurities. The second grinding can fully disperse the ferric phosphate dihydrate agglomerate particles generated by conversion, and the PO coated in the agglomerate particles4 3-Releasing to facilitate the next step of water washing to remove PO4 3-The iron-phosphorus ratio of the product is improved as much as possible, the purity of the lithium iron phosphate is finally improved, and the charge and discharge capacity of the lithium iron phosphate is further improved. Meanwhile, the grinded particles become uniform and fine, and are mostly soft agglomerated although being agglomerated in the subsequent calcining process, so that the grinding dispersion is easy, and the grinding efficiency in the subsequent lithium iron production can be greatly improved. In addition, because the particles are uniform and fine and easy to sinter, finally obtained lithium iron phosphate crystal grains are also more uniform and fine, abnormal large crystal grains are not easy to generate, the migration distance of lithium ions in the charging and discharging process is shortened, the lithium ions are favorably embedded and separated, and the charging and discharging capacity of the lithium iron phosphate is further improved. (3) Glucose, graphite and carbon nanotubes are used as coating carbon sources. The coating carbon is composed of two parts, an amorphous carbon coating obtained by carbonizing glucose clings to the surface of lithium iron phosphate particles, and graphite and carbon nano tubes are wound and coated on the surface of the particles or filled in gaps among the particles. The amorphous carbon is tightly attached to the surfaces of the lithium iron phosphate particles, so that the contact area is greatly increased, and the transfer of electrons is facilitated; the carbon nano tube and the graphite can effectively fill gaps among particles, and the isolated particles are connected into a whole, so that electrons can be transferred to an external circuit. Adding MoO3The doping is carried out, so that the conductivity of the material is further improved;
the high-capacity high-compaction-density high-iron-phosphorus-ratio nano lithium iron phosphate provided by the invention has an iron-phosphorus ratio (the achievable range is 0.985-1.0) and a compaction density (the achievable range is 2.45-2.55 g/cm)3) The battery has the advantages of high gram capacity (the achievable range is more than or equal to 157mAh/g), excellent battery performance, and charge-discharge tests show that the first charge specific capacity under the multiplying power of 0.1C is 161-165 mAh/g, the first discharge specific capacity is 157-161 mAh/g, and the capacity retention rate is not lower than 98% after 100 cycles.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.
Fig. 1 is an SEM image of lithium iron phosphate provided in embodiment 1 of the present invention;
fig. 2 is an XRD pattern of lithium iron phosphate provided in embodiment 1 of the present invention;
fig. 3 is a charge-discharge curve diagram (full cell) of lithium iron phosphate provided in embodiment 1 of the present invention;
fig. 4 is a flowchart of a method for preparing nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio according to an embodiment of the present invention.
Detailed Description
The embodiments of the present invention will be described in detail below with reference to specific embodiments and examples, and the advantages and various effects of the embodiments of the present invention will be more clearly apparent therefrom. It will be understood by those skilled in the art that the present embodiments and examples are illustrative of the present invention and are not to be construed as limiting the present invention.
First, it is stated that the term "and/or" appearing herein is merely one type of association that describes an associated object, meaning that three types of relationships may exist, e.g., a and/or B may represent;
it is noted that, in this document, relational terms such as "first" and "second," and the like, may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions.
In order to solve the technical problems, the embodiment of the invention provides the following general ideas:
according to an exemplary embodiment of the present invention, there is provided a method for preparing nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio, as shown in fig. 4, the method includes:
step S1, mixing a phosphorus salt solution and an iron salt solution for oxidation reaction to obtain an oxidation slurry;
step S2, carrying out first grinding on the oxidized slurry to obtain fine-grain-size oxidized slurry, wherein the fine-grain-size oxidized slurry meets the following requirements: d99 is less than or equal to 1 mu m, and D50 is less than or equal to 300 nm;
step S3, after the fine particle size slurry is subjected to first washing, pure water and phosphoric acid are added for conversion reaction, and conversion slurry is obtained;
step S4, adjusting the pH value of the conversion slurry to 6-8, and then carrying out second grinding to obtain a fine-particle-size conversion slurry, wherein the fine-particle-size conversion slurry meets the following requirements: d99 is less than or equal to 1 mu m, and D50 is less than or equal to 300 nm;
step S5, carrying out second washing on the fine-particle size conversion slurry, drying and calcining to obtain anhydrous iron phosphate;
step S6, mixing the anhydrous ferric phosphate with lithium carbonate, glucose and pure water, and then adding MoO3And uniformly mixing graphite and the carbon nano tube to obtain a mixture, and then sequentially grinding, drying, sintering and crushing the mixture in an inert atmosphere to obtain the lithium iron phosphate material.
In the technical proposal, the device comprises a base,
the reason why the first grinding into the fine particle size oxidized slurry is performed in step S2 is: the generated amorphous iron phosphate can be fully dispersed, the agglomeration is removed, and impurity ions wrapped in the agglomerated particles are released, so that the next step of water washing is facilitated, and the impurity ions are removed as far as possible; meanwhile, the ground particles become uniform and fine, which is beneficial to the subsequent conversion reaction and improves the conversion rate and the conversion rate. The fine particle size oxidation slurry meets the following requirements: d99 is less than or equal to 1 mu m, and D50 is less than or equal to 300 nm. If D99 is larger than 1 mu m and D50 is larger than 300nm, the slurry has large particles, and the large particles are wrapped with impurity ions and cannot be removed by washing, so that the content of the impurity ions is higher; meanwhile, the ion migration path inside the large particles is long, and the subsequent conversion rate and conversion rate are influenced.
The reason why the pH adjustment and the second grinding are performed in the step S4 is:
because excessive phosphoric acid is added in the conversion process, the excessive phosphoric acid can be converted into ammonium phosphate by adding the pH regulator, and the subsequent washing and impurity removal are facilitated. If the pH is less than 6, the phosphoric acid in the slurry is not completely converted, the subsequent washing rate is influenced, and the consumption of pure water for washing is increased; if the pH value is more than 8, the use amount of pure water for washing is increased, the washing time is prolonged, the waste of a pH regulator is caused, and the cost is further increased;
the second grinding has the similar function as the first grinding, and can fully disperse the ferric phosphate dihydrate agglomerate particles generated by conversion and wrap PO inside the agglomerate particles4 3-Releasing to facilitate the next step of water washing to remove PO4 3-The iron-phosphorus ratio of the product is improved as much as possible, the purity of the lithium iron phosphate is finally improved, and the charge and discharge capacity of the lithium iron phosphate is further improved. Meanwhile, the grinded particles become uniform and fine, and are mostly soft agglomerated although being agglomerated in the subsequent calcining process, so that the grinding dispersion is easy, and the grinding efficiency in the subsequent lithium iron production can be greatly improved. In addition, because the particles are uniform and fine and easy to sinter, finally obtained lithium iron phosphate crystal grains are also more uniform and fine, abnormal large crystal grains are not easy to generate, the migration distance of lithium ions in the charging and discharging process is shortened, the lithium ions are favorably embedded and separated, and the charging and discharging capacity of the lithium iron phosphate is further improved.
The invention, through the steps S1-S5, prepares the obtained anhydrous ferric phosphate: the iron-phosphorus ratio is high, the primary crystal grains are fine, the secondary particles are uniform, the sphericity is good, and the compaction density and the charge-discharge capacity of the lithium iron can be improved;
in the step S6, in the above step,
glucose, graphite and carbon nanotubes are used as a carbon source for coating. The coating carbon is composed of two parts, an amorphous carbon coating obtained by carbonizing glucose clings to the surface of lithium iron phosphate particles, and graphite and carbon nano tubes are wound and coated on the surface of the particles or filled in gaps among the particles. The amorphous carbon is tightly attached to the surfaces of the lithium iron phosphate particles, so that the contact area is greatly increased, and the transfer of electrons is facilitated; the carbon nano tube and the graphite can effectively fill gaps among particles, and the isolated particles are connected into a whole, so that electrons can be transferred to an external circuit. In the charging and discharging process, electrons are collected through the amorphous carbon coating and then are transmitted by the carbon nano tube with better conductivity and the graphite, so that the electronic conductivity of the material can be effectively improved.
② adding MoO3Doping is carried out, Mo occupies the position of iron in the lithium iron phosphate, and defects such as vacancies are formed, so that the migration rate of lithium ions is improved, the electronic conductivity of the material is also improved, and the electrochemical performance is further greatly improved;
because the doping and carbon coating forms are optimized, and the conductivity is improved, the carbon content of the lithium iron phosphate is lower than that of the conventional process (the carbon content is less than 1.2 percent), so that the compaction density is further improved; the ion conductivity and the electron conductivity are improved by doping and cladding, and finally the electrical properties of the lithium iron phosphate material, such as gram capacity, low-temperature performance, rate performance and the like, are improved; in addition, as the anhydrous ferric phosphate particles are fine and uniform and are mostly soft and agglomerated, the slurry is easy to grind, and the grinding time is greatly shortened;
in conclusion, the nano lithium iron phosphate prepared by the method has the advantages of high capacity, high compaction density and high iron-phosphorus ratio.
As an alternative, in step S1,
in the oxidation reaction, the mass ratio of the phosphorus element in the phosphorus salt solution to the iron element in the iron salt solution is (1-1.05): 1. The ratio can ensure that phosphate radical and ferrous ion react according to the ratio of 1:1, and meanwhile, the phosphate is slightly excessive, and ferric salt is fully reacted.
Specifically, a phosphorus salt solution is dripped into an iron salt solution, the stirring speed is 180-220 rpm, and the reaction time is 0.5-3 h, so that amorphous ferric phosphate oxidation slurry is generated;
as a specific embodiment, it is possible to use,
the preparation method of the iron salt solution comprises the following steps: FeSO (ferric oxide) is added4Or FeSO4 7H2Dissolving O in pure water, filtering to obtain iron salt solution, and FeSO in the iron salt solution4The concentration of (A) is 0.5-2 mol/L;
the preparation method of the phosphorus salt solution comprises the following steps: dissolving MAP or DAP with pure water, adjusting the pH value to 6-8 with ammonia water or phosphoric acid, adding hydrogen peroxide, and uniformly stirring to obtain a phosphorus salt solution, wherein the content of P in the phosphorus salt solution is 1-4 mol/L, and the mass concentration of the hydrogen peroxide in the phosphorus salt solution is 0.55-0.65 times of the content of P;
in other embodiments, the solution of the phosphorous salt and the solution of the iron salt may be obtained in other ways.
As an alternative, in step S3,
the first washing includes: and filtering the slurry with the fine particle size and washing with pure water, wherein the washing end point is that the conductivity of washing water is less than or equal to 1 mS/cm. Thus being beneficial to fully removing impurities, if the conductivity of the washing water is more than 1mS/cm, the impurity ions are not washed, which not only influences the content of impurity elements in the finished product, but also influences the conversion rate of the conversion process;
in the conversion reaction, pure water and phosphoric acid are added to control the pH value of the fine particle size slurry to be 1.0-2.2, and the solid content is 10-20 wt%. The arrangement is favorable for conversion of amorphous ferric phosphate to ferric phosphate dihydrate, and if the pH value is less than 1, the ferric phosphate dihydrate is partially dissolved, so that the loss of the ferric phosphate is caused; if the crystal form is more than 2.2, the crystal form transformation of amorphous iron phosphate is not facilitated; if the solid content is less than 10 wt%, the conversion rate and the productivity are influenced, and if the solid content is more than 20 wt%, the stirring is not uniform, and the uniformity of the product is influenced;
the temperature of the conversion reaction is 90-100 ℃, and the time of the conversion reaction is 1-4 h. The reaction temperature is favorable for overcoming the energy barrier of crystal form transformation, the crystal form transformation occurs, and amorphous iron phosphate is fully transformed into ferric phosphate dihydrate crystals.
As an alternative, in step S5,
the dry calcination comprises: drying at 120-180 ℃ to remove free water, and calcining at 550-650 ℃ for 2-4 h to remove crystal water.
Under the conditions, the free water and the crystal water in the dihydrate ferric phosphate can be completely removed.
As an alternative, in step S6,
the mass ratio of the anhydrous iron phosphate to the lithium carbonate is 1: (0.5 to 0.55); in the process of generating the lithium iron phosphate by the reaction, the lithium ions are slightly excessive, so that the iron phosphate is ensured to fully react.
The mass of the glucose is 7-8.5 wt% of that of the anhydrous ferric phosphate; calcining the mixture in an inert atmosphere to carbonize glucose to generate amorphous carbon, wherein one part of the amorphous carbon is used as a reducing agent to reduce ferric iron into bivalent iron, and the rest of the amorphous carbon is used as a conductive agent to coat the surfaces of the lithium iron phosphate particles; if the addition amount of the glucose is less than 7%, the amount of the coated carbon is too low, so that the conductivity of the product is influenced, and if the addition amount of the glucose is more than 8.5%, the amount of the coated carbon is too high, so that the compaction density and the specific surface area of the product are influenced;
the adding amount of the pure water is 40-45 wt% of solid content; if the solid content is less than 40 wt%, the productivity of the equipment is wasted, and if the solid content is more than 45 wt%, the grinding difficulty is increased, and the grinding effect is influenced;
the Mo doping amount is 0.1-0.2 wt% of the theoretical mass of the lithium iron phosphate according to the mass of Mo element, and MoO3The addition amount of the lithium iron phosphate is too small, the improvement degree of the material performance is limited, the doping amount is too large, the original structure of the lithium iron phosphate can be damaged, and the stability of the lithium iron phosphate is reduced;
the mass of the graphite is 0.1-0.2 wt% of the theoretical mass of the lithium iron phosphate. The doping amount of the graphite is too small, so that the improvement of the electrical conductivity of the material is limited, and if the doping amount of the graphite is too large, the compaction density of the material is reduced, so that the processing performance of the positive electrode material in the using process is influenced;
the mass of the carbon nano tube is 0.1-0.2 wt% of the theoretical weight of the lithium iron phosphate. The carbon nano tube has an influence mechanism similar to that of graphite, the doping amount is too small, the improvement on the electrical conductivity of the material is limited, the compaction density of the material is reduced when the doping amount is too large, the specific surface area is too large, and the processability of the positive electrode material in the using process is influenced;
according to another typical embodiment of the invention, the method for preparing the nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio is also provided.
The preparation method of the nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio according to the present application will be described in detail below with reference to examples, comparative examples and experimental data.
The following examples are provided only for illustrating the present invention and are not intended to limit the scope of the present invention. All other embodiments obtained by a person skilled in the art based on the specific embodiments of the present invention without any inventive step are within the scope of the present invention.
In the examples of the present invention, all the raw material components are commercially available products well known to those skilled in the art, unless otherwise specified; in the examples of the present invention, unless otherwise specified, all technical means used are conventional means well known to those skilled in the art. In the examples of the present invention, the raw materials used were all conventional commercially available products.
Example 1
The embodiment of the invention provides a preparation method of nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio, which comprises the following steps:
firstly, preparing anhydrous ferric phosphate
Step S1, dropwise adding a phosphorus salt solution into an iron salt solution, wherein the mass ratio of phosphorus to iron is (1-1.05): 1, the stirring speed is 200rpm, and the reaction time is 0.5-3 h, so that amorphous iron phosphate is generated, and oxidation slurry is obtained; wherein the content of the first and second substances,
the preparation method of the iron source comprises the following steps: FeSO (ferric oxide) is added4Or FeSO4·7H2Dissolving O in pure water, filtering to obtain iron salt solution, and FeSO in the iron salt solution4The concentration of (A) is 0.5-2 mol/L;
the preparation method of the phosphorus source comprises the following steps: dissolving MAP or DAP with pure water, adjusting the pH value to 6-8 with ammonia water or phosphoric acid, adding hydrogen peroxide, and uniformly stirring to obtain a phosphorus salt solution, wherein the P content in the phosphorus salt solution is 1-4 mol/L, and the concentration (substance quantity concentration) of the hydrogen peroxide in the phosphorus salt solution is 0.55-0.65 times of the P content;
step S2, grinding the oxidized slurry by a sand mill, and controlling D99 to be less than or equal to 1 μm and D50 to be 300 nm;
step S3, filtering the ground slurry by using a filter press and washing the slurry by using pure water, wherein the washing end point is that the conductivity of washing water is less than or equal to 1 mS/cm;
step S4, adding pure water and phosphoric acid into the obtained filter cake, fully stirring, controlling the pH value of the slurry to be 1.0-2.2 and the solid content to be 10% -20%, raising the temperature of the slurry to 90-100 ℃, preserving the temperature for 1-4 h, and converting amorphous iron phosphate into ferric phosphate dihydrate to obtain converted slurry;
step S5, adding a pH regulator into the obtained conversion slurry, controlling the pH of the slurry to be 7, stirring for 0.5-1 h at a stirring speed of 200rpm, wherein the pH regulator is one or more of ammonia water, ammonium carbonate and ammonium bicarbonate; secondly, grinding the obtained slurry by using a sand mill for the second time, wherein D99 is less than or equal to 1 mu m, and D50 is 300 nm;
s6, filtering the obtained slurry by using a filter press, washing a filter cake by using pure water to carry out secondary washing for removing impurities, wherein the washing end point is that the conductivity of washing water is less than or equal to 200 mu S/cm; then drying and calcining: drying the obtained filter cake at 120-180 ℃ to remove free water, calcining at 550-650 ℃ for 2-4 h to remove crystal water, and obtaining anhydrous iron phosphate;
secondly, preparing lithium iron phosphate by using anhydrous iron phosphate
The anhydrous iron phosphate obtained above was mixed with lithium carbonate (mass ratio of 1: 0.51), glucose (7.75% by mass of the anhydrous iron phosphate), and pure water (solid content: 42%), and MoO was added thereto3Doping (Mo is 0.20 percent of the theoretical weight of the lithium iron phosphate), adding graphite (0.20 percent of the theoretical weight of the lithium iron phosphate) and a carbon nano tube (0.20 percent of the theoretical weight of the lithium iron phosphate), grinding by a sand mill, spray drying, sintering in an inert atmosphere, and carrying out jet milling to obtain a lithium iron phosphate material;
example 2
In example 2, the pH of the conversion slurry was adjusted to 6, and the other steps were the same as in example 1.
Example 3
In example 3, the pH of the conversion slurry was adjusted to 8, and the other steps were the same as in example 1.
Example 4
In example 4, the first and second post-milling D50 were 250nm, and the other steps were the same as in example 1.
Example 5
In example 5, the amount of glucose added was 7% by mass of anhydrous ferric phosphate, and the procedure was the same as in example 1.
Example 6
In example 6, the amount of glucose added was 8.5% by mass of anhydrous ferric phosphate, and the procedure was the same as in example 1.
Example 7
In example 7, MoO3Mo in the doping accounts for 0.10 percent of the theoretical weight of the lithium iron phosphate, and other steps are the same as those in the embodiment 1.
Example 8
In example 8, the amount of graphite added was 0.10% of the theoretical weight of lithium iron phosphate, and the other steps were the same as in example 1.
Example 9
In example 9, the amount of carbon nanotubes added was 0.10% of the theoretical weight of lithium iron phosphate, and the other steps were the same as in example 1.
Example 10
In example 10, MoO3In the doping, the Mo accounts for 0.15% of the theoretical weight of the lithium iron phosphate, the graphite additive accounts for 0.15% of the theoretical weight of the lithium iron phosphate, and the carbon nanotube additive accounts for 0.15% of the theoretical weight of the lithium iron phosphate, and other steps are the same as those in example 1.
Comparative example 1
In this comparative example, the pH was not adjusted prior to the second grind; the other steps were the same as in example 1.
Comparative example 2
In this comparative example, the first grinding and the second grinding were not performed; the other steps were the same as in example 1.
Comparative example 3
In this comparative example, the particle size D50 of both the first grind and the second grind was 500 nm; the other steps were the same as in example 1.
Comparative example 4
In this comparative example, the second grinding was not performed; the other steps were the same as in example 1.
Comparative example 5
In this comparative example, the first grinding was not performed; the other steps were the same as in example 1.
Comparative example 6
In this comparative example, the first milling, and the pH adjustment and second milling were not performed; the other steps were the same as in example 1.
Comparative example 7
In the comparative example, the addition amount of glucose was 10% by mass of anhydrous iron phosphate; the other steps were the same as in example 1.
Comparative example 8
In this comparative example, no MoO was added3(ii) a The other steps were the same as in example 1.
Comparative example 9
In this comparative example, no MoO was added3Graphite and carbon nanotubes; the other steps were the same as in example 1.
Comparative example 10
Comparative example 10 lithium iron phosphate prepared by a conventional process without first milling, pH adjustment and second milling, and without addition of MoO3Graphite and carbon nano tubes, wherein the addition amount of glucose is 12% of the mass of the anhydrous iron phosphate; the other steps were the same as in example 1.
Experimental example 1
1. The process parameters for each example and comparative example are tabulated below in table 1, and the tabulated experimental conditions are the same as in example 1.
TABLE 1
2. The performance of the anhydrous iron phosphates of examples 1 to 4 and comparative examples 1 to 6 was examined, and the examination results are shown in table 2:
TABLE 2
From the data in table 2, it can be seen that:
(1) examples 1-3, comparative example 1, comparative analysis of the Effect of pH adjustment on the Performance of Anhydrous iron phosphate before Secondary grinding
Examples 1-3, comparative example 1: after the pH value is adjusted (the optimal range is 6-8), the iron-phosphorus ratio of the anhydrous iron phosphate is obviously increased (more than 0.985), and the pH value of the anhydrous iron phosphate is increased;
(2) examples 1 and 4, comparative examples 2 to 5, comparative analysis of the influence of the first and second mills on the performance of anhydrous iron phosphate
Examples 1, 4, comparative examples 2 to 5: the content of impurity elements (mainly soluble impurities, S, Na, K and the like) of the anhydrous ferric phosphate is reduced after the grinding, and the finer the grinding is, the lower the impurity content is; after secondary grinding, the iron-phosphorus ratio of the anhydrous iron phosphate is increased, the D50 is reduced, and the BET is increased;
(3) comparative example 6 is anhydrous iron phosphate prepared by a conventional process, and no primary grinding, secondary grinding and pH adjustment are performed, so that the iron-phosphorus ratio is lower, the content of impurity elements is higher, the particle size is larger, and the performance is poorer;
2. the performance of the lithium iron phosphate of examples 1, 5 to 10 and 7 to 10 was then examined, and the results are shown in table 3:
TABLE 3
From the data in table 3, it can be seen that:
(1) examples 1, 5 and 6, comparative example 7, comparative analysis of the effect of the amount of glucose on the performance of lithium iron phosphate
Examples 1, 5, 6, comparative example 7: with the increase of the addition of glucose, the carbon content of the lithium iron phosphate is increased, the BET is increased, the charge-discharge capacity is increased, but the compaction density is reduced;
(2) examples 1, 7-10, comparative examples 8, 9, comparative analysis of the impact of Mo, graphite and carbon nanotubes on lithium iron phosphate performance
Examples 1, 7 to 10, comparative examples 8, 9: mo, graphite and carbon nano tube doping can effectively improve the gram capacity of the lithium iron phosphate, but the compaction density is slightly reduced;
(3) the comparative example 10 is lithium iron phosphate prepared by a conventional process, and compared with the patent example, the iron-phosphorus ratio, the compaction density and the gram volume are lower, and the content of impurity elements is higher;
description of the drawings:
as shown in fig. 1, an SEM image of lithium iron phosphate provided in embodiment 1 of the present invention shows that lithium iron phosphate particles have a high sphericity and a uniform and fine size, carbon coating layers are provided on the surfaces of the particles, carbon nanotubes are wrapped around the particles, and graphite is filled in gaps between the particles.
The XRD pattern of the lithium iron phosphate provided in example 1 of the present invention is shown in fig. 2, which has no impurity peak in the XRD pattern, matches with the standard card of lithium iron phosphate, and has a high and narrow diffraction peak, indicating that the crystallinity of lithium iron phosphate is high.
Fig. 3 shows a charge-discharge curve diagram of lithium iron phosphate provided in embodiment 1 of the present invention, and it can be seen that the discharge capacity is 43359mAh, and the discharge plateau is 3.1V to 3.2V. And according to the battery cell processing data, the dressing amount of the positive plate is 290g of lithium iron phosphate, and the gram capacity exertion of the positive material is 149.5 mAh/g.
Finally, it should also be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including preferred embodiments and all such alterations and modifications as fall within the scope of the invention.
It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.
Claims (10)
1. A preparation method of nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio is characterized by comprising the following steps:
mixing a phosphorus salt solution and an iron salt solution for oxidation reaction to obtain an oxidation slurry;
carrying out first grinding on the oxidized slurry to obtain fine-grain-size oxidized slurry, wherein the fine-grain-size oxidized slurry meets the following requirements: d99 is less than or equal to 1 mu m, and D50 is less than or equal to 300 nm;
after the fine-particle size slurry is subjected to first washing, adding pure water and phosphoric acid to perform conversion reaction to obtain conversion slurry;
adjusting the pH value of the conversion slurry to 6-8, and then carrying out second grinding to obtain conversion slurry with a fine particle size, wherein the conversion slurry with the fine particle size meets the following requirements: d99 is less than or equal to 1 mu m, and D50 is less than or equal to 300 nm;
carrying out second washing on the fine-particle size conversion slurry, drying and calcining to obtain anhydrous iron phosphate;
mixing the anhydrous iron phosphate with lithium carbonate, glucose and pure water, and then adding MoO3And uniformly mixing graphite and the carbon nano tube to obtain a mixture, and then sequentially grinding, drying, sintering and crushing the mixture in an inert atmosphere to obtain the lithium iron phosphate material.
2. The preparation method of the nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio according to claim 1, wherein in the oxidation reaction, the mass ratio of the phosphorus element in the phosphorus salt solution to the iron element in the iron salt solution is (1-1.05): 1.
3. The method for preparing nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio according to claim 1, wherein the first washing comprises the following steps: and filtering the slurry with the fine particle size and washing with pure water, wherein the washing end point is that the conductivity of washing water is less than or equal to 1 mS/cm.
4. The preparation method of the nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio according to claim 1, wherein in the conversion reaction, pure water and phosphoric acid are added to control the pH value of the slurry with fine particle size to be 1.0-2.2, and the solid content is 10-20 wt%.
5. The preparation method of the nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio according to claim 1, wherein the temperature of the conversion reaction is 90-100 ℃, and the time of the conversion reaction is 1-4 h.
6. The method for preparing nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio according to claim 1, wherein a pH regulator is adopted for pH regulation, and the pH regulator is at least one of ammonia water, ammonium carbonate and ammonium bicarbonate.
7. The method for preparing nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio according to claim 1, wherein the second washing comprises the following steps: and filtering the fine particle size conversion slurry, and washing with pure water, wherein the washing end point is that the conductivity of washing water is less than or equal to 200 mu S/cm.
8. The method for preparing nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio according to claim 1, wherein the drying and calcining comprises the following steps: drying at 120-180 ℃ to remove free water, and calcining at 550-650 ℃ for 2-4 h to remove crystal water.
9. The method for preparing nano lithium iron phosphate with high capacity, high compaction density and high iron-phosphorus ratio according to claim 1, wherein the anhydrous iron phosphate is mixed with lithium carbonate, glucose and pure water, and MoO is added after the anhydrous iron phosphate is mixed with the lithium carbonate, the glucose and the pure water3And uniformly mixing the graphite and the carbon nano tube to obtain a mixture, which specifically comprises the following steps:
the anhydrous iron phosphate is mixed with lithium carbonate, glucose and pure water, wherein the mass ratio of the anhydrous iron phosphate to the lithium carbonate is 1: (0.5 to 0.55); the mass of the glucose is 7-8.5 wt% of that of the anhydrous ferric phosphate; the adding amount of the pure water is 40-45 wt% of solid content;
post-addition of MoO3Mixing graphite and carbon nano tube uniformly to obtain a mixture, wherein the MoO3The addition amount of the carbon nano tube is determined according to the condition that the mass of the Mo element is 0.1-0.2 wt% of the theoretical mass of the lithium iron phosphate, the mass of the graphite is 0.1-0.2 wt% of the theoretical mass of the lithium iron phosphate, and the mass of the carbon nano tube is 0.1-0.2 wt% of the theoretical weight of the lithium iron phosphate.
10. A high capacity high compacted density high iron to phosphorus ratio nano lithium iron phosphate prepared by the method of any one of claims 1 to 9.
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