CN116190640B - Carbon-coated pyrophosphoric acid polyanion composite material and preparation method and application thereof - Google Patents
Carbon-coated pyrophosphoric acid polyanion composite material and preparation method and application thereof Download PDFInfo
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Abstract
The invention discloses a carbon-coated pyrophosphoric acid polyanion composite material, and a preparation method and application thereof. The composite material consists of pyrophosphoric acid polyanion and a carbon layer coated on the surface of the pyrophosphoric acid polyanion in situ; the structural general formula of the pyrophosphoric acid polyanion is Na 3.12‑y Fe 2.44‑x M x (P 2 O 7 ) 2 Wherein x is more than or equal to 0 and less than or equal to 0.5, y is more than or equal to 0 and less than or equal to 1, and M is one of Mn, V, ti, mg, al, cu, Y and Zr. The composite material is based on the synergistic effect among the constituent elements, and the electrochemical performance of the material is greatly optimized on the premise of ensuring that the material has good stability. The material is fed once, and the stable pure phase compound is obtained through high-energy ball milling and calcination, and the method has the advantages of simple process, low cost and the like, and is suitable for industrial production. The sodium ion battery prepared by taking the composite material as the positive electrode material has a higher voltage platform, good stability and excellent electrochemical performance.
Description
Technical Field
The invention relates to a pyrophosphoric acid polyanion composite material, in particular to a carbon-coated pyrophosphoric acid polyanion composite material, and a preparation method and application thereof, and belongs to the technical field of sodium ion batteries.
Background
In order to achieve the goal of carbon reduction, there is an urgent need to efficiently utilize renewable clean energy sources such as solar energy, wind energy, tidal energy, and geothermal energy to reduce carbon emissions. However, the renewable energy sources often have the characteristics of periodicity, intermittence and the like, and the renewable energy sources can be reasonably utilized after being integrated and converted by a large-scale energy storage device. For this reason, there is an urgent need to develop energy storage technologies that are advantageous in terms of both resources and costs. In the existing energy storage technology, the secondary ion battery is considered as an ideal choice of a large-scale energy storage technology due to high energy density, stable operation and low maintenance cost.
Compared with a lithium ion battery, the sodium ion battery has the advantages of abundant sodium resources, relatively low cost, good safety and large-scale application. The key point of sodium ion battery technology is mainly in development of electrode materials and electrolyte. Although the sodium ion cathode materials reported so far are of a relatively rich variety, mainly including layered oxides, prussian blue analogues, and polyanionic compounds, there are few materials that exhibit good electrochemical properties, and these materials also face some problems during commercial applications. From the aspects of manufacturing cost, sodium storage performance, environmental protection and the like, na 3.12 Fe 2.44 (P 2 O 7 ) 2 Is considered as the most potential positive electrode material of sodium ion batteries. It has stable iron-based phosphate structure, low cost and high theoretical capacity (117 mAh g) -1 ) And thus receives a great deal of attention. However Na is 3.12 Fe 2.44 (P 2 O 7 ) 2 The electron conductivity of (c) is low, resulting in difficult realization of theoretical capacity and poor rate performance. At the same time, for Na 3.12 Fe 2.44 (P 2 O 7 ) 2 The research on the mechanism of sodium intercalation and deintercalation is relatively lack, and deep understanding is needed. Therefore, a preparation method which has simple process, low cost and easy scale-up is developed to obtain Na with excellent electrochemical performance 3.12 Fe 2.44 (P 2 O 7 ) 2 Positive electrode materials are imperative.
Disclosure of Invention
A first object of the present invention, which addresses the problems of the prior art, is to provide a carbon-coated pyrophosphate polyanion composite material. The composite material is based on the synergistic effect among the constituent elements, and the electrochemical performance of the material is greatly optimized on the premise of ensuring that the material has good stability; the invention provides abundant transport channels for Na ions by limiting the molar ratio of Na element to metal doped element in the material, improves the multiplying power performance and the circulation stability of the material, and further strengthens the chemical stability and the thermal stability of the material by the in-situ generated conductive carbon layer, thereby endowing the material with certain heat resistance and solvent resistance.
The second object of the invention is to provide a preparation method of the carbon-coated pyrophosphoric acid polyanion composite material, which adopts one-time feeding, fully and uniformly mixes raw materials through high-energy ball milling on one hand, and changes the activation energy of the surface of the material on the other hand, thereby being convenient for forming pure-phase compounds in the subsequent sintering process and effectively improving the purity of the material; the method has the advantages of simple process, low cost and the like, and is suitable for industrial production.
The third object of the invention is to provide an application of the carbon-coated pyrophosphoric acid polyanion composite material as a positive electrode active material of a sodium ion battery to prepare the sodium ion battery. The sodium ion battery prepared based on the composite material provided by the invention has the working voltage of about 3.0V and good long-cycle stability through test, and lays a foundation for promoting the industrialization of the sodium ion battery.
In order to achieve the technical aim, the invention provides a preparation method of a carbon-coated pyrophosphoric acid polyanion composite material, which comprises the steps of dispersing raw materials comprising a sodium source, an iron source, a phosphorus source, a doping element source, a carbon source and a reducing agent in a solvent, and sequentially performing high-energy ball milling, drying, tabletting and sintering to obtain the carbon-coated pyrophosphoric acid polyanion composite material; the molar ratio of the sodium element, the iron element and the phosphorus element in the raw materials is 3-3.3: 1.8-2.5: 3.9-4.1; the molar ratio of the metal element in the raw materials to the reducing agent is 2-3:3; the sintering process conditions are as follows: in an inert/reducing atmosphere, the temperature is 400-600 ℃ and the time is 6-20 h; the molar ratio of the sodium element to the doping element is 3-3.3: 0-0.5; the doping element is one of Mn, V, ti, mg, al, cu, Y and Zr.
Na provided by the invention 3.12-y Fe 2.44-x M x (P 2 O 7 ) 2 The composite material has the advantages of iron-based phosphate, and the theoretical capacity of the composite material is as high as 117mAh g -1 The volume expansion is as low as 4%. However, the low electron conductivity and the formation of impurity phases can seriously affect the electrochemical performance. The invention prepares the composite material by high-energy ball millingOn the one hand, all the precursor salts can be uniformly mixed, and a fine and uniform precursor mixture can be quickly obtained; on the other hand, the precursor surface can be energized, so that the element doping and the combination in the subsequent sintering process are convenient, the generation of impurity phases is reduced as much as possible, and a pure-phase product is obtained. The preparation method provided by the invention is simple, convenient and feasible, and the obtained product has high yield and is convenient for industrial production.
As a preferable scheme, the conditions of the high-energy ball milling are as follows: the rotating speed is 300-500 r/min, and the time is 6-24 h.
As a preferred embodiment, the sodium source is one of trisodium phosphate, monosodium phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium oxide, and sodium peroxide. Further preferably, the sodium source is sodium acetate.
As a preferred embodiment, the iron source is one of iron acetylacetonate, ferrous acetylacetonate, iron powder, iron nitrate, ferrous nitrate, iron sulfate, ferrous sulfate, ferric oxide, ferrous oxide, ferric oxalate, ferrous oxalate, ferric acetate, and ferric phosphate. Further preferably, the iron source is ferrous acetylacetonate.
As a preferred embodiment, the doping metal source is at least one of an inorganic salt, an organic salt, a hydroxide, and an oxide containing a doping metal element.
As a preferred embodiment, the phosphorus source is one of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, ammonium dihydrogen phosphate and sodium pyrophosphate. Further preferably, the phosphorus source is phosphoric acid.
As a preferred embodiment, the carbon source is one of graphene, carbon nanotubes, conductive carbon black, sucrose, oxalic acid, glucose, ascorbic acid, polyvinylpyrrolidone and citric acid. Further preferably, the carbon source is a carbon nanotube.
As a preferred embodiment, the reducing agent is one of citric acid, oxalic acid and adipic acid. Further preferably, the reducing agent is citric acid.
As a preferred embodiment, the solvent is one of deionized water, ethanol, isopropanol, and acetone.
As a preferable scheme, the high-energy ball milling is wet ball milling, and the ball-to-material ratio is 5-10:1.
As a preferred embodiment, the drying means is one of forced air drying, vacuum drying and freeze drying.
As a preferable scheme, the pressure in the tabletting process is 4-20 MPa. The tabletting sintering process is beneficial to migration of each element, so that the element distribution is more uniform, and the compactness and other performances of the material are positively influenced.
As a preferred embodiment, the sintering process conditions are: and in an inert/reducing atmosphere, heating to 400-600 ℃ at a heating rate of 2-10 ℃/min, and performing constant-temperature sintering for 6-20 h.
As a preferred embodiment, the inert atmosphere is a nitrogen atmosphere and/or an argon atmosphere.
As a preferable scheme, the reducing atmosphere is nitrogen-hydrogen mixed gas or argon-hydrogen mixed gas, wherein the volume fraction of hydrogen in the mixed gas is 3-20%.
The invention also provides a carbon-coated pyrophosphoric acid polyanion composite material, which is prepared by the preparation method of any one of the above; the composite material consists of pyrophosphoric acid polyanion and a carbon layer coated on the surface of the pyrophosphoric acid polyanion in situ; the structural general formula of the pyrophosphoric acid polyanion is Na 3.12-y Fe 2.44-x M x (P 2 O 7 ) 2 Wherein x is more than or equal to 0 and less than or equal to 0.5, y is more than or equal to 0 and less than or equal to 1, and M is a doped metal element.
The pyrophosphoric acid polyanion material Na provided by the invention 3.12 Fe 2.44 (P 2 O 7 ) 2 Is composed of Fe with common angle 2 P 2 O 22 And Fe (Fe) 2 P 4 O 20 The basic units are formed by central symmetry connection, provide a wide one-dimensional migration path for sodium ion deintercalation, and are obviously superior to NaFeP 2 O 7 And Na (Na) 2 FeP 2 O 7 A material. The doped element M can cause crystallizationLattice distortion causes Na 3.12-y Fe 2.44-x M x (P 2 O 7 ) 2 The lattice contraction of the material improves the intrinsic electron conductivity, thereby promoting the obvious improvement of the material in the aspects of rate performance and cycle performance.
As a preferable scheme, the composite material crystal system is a triclinic system, and the space group is P-1. It belongs to the crystal system with the lowest symmetry degree, the axial length a is not equal to b not equal to c, and the axial angle alpha is not equal to beta is not equal to gamma is not equal to 90 degrees.
As a preferable scheme, the in-situ coated carbon layer accounts for 3-15% of the total mass of the composite material.
The invention also provides application of the carbon-coated pyrophosphoric acid polyanion composite material as an anode active material of a sodium ion battery to prepare the sodium ion battery. The average operating voltage of the carbon-coated pyrophosphoric acid polyanion composite material was about 3.0V, as tested in a potential window of 1.5-4.0V, and the initial specific discharge capacity at 5C was 79mAh g -1 After 500 cycles, the capacity retention rate is as high as 97.8%. In addition, the carbon-coated pyrophosphoric acid polyanion composite material has excellent rate capability, and can reach 58mAh g even at 30C -1 Is a very high capacity of (a). (1c=117 mA g -1 ）
Compared with the prior art, the technical scheme of the invention has the following beneficial technical effects:
1) The composite material provided by the invention is based on the synergistic effect among all the constituent elements, and on the premise of ensuring that the material has good stability, the electrochemical performance of the material is greatly optimized, and the bulk phase structure is regulated and controlled through element doping, so that the problem that the theoretical capacity of the composite material cannot be realized due to low electronic conductivity is solved; the invention provides abundant transport channels for Na ions by limiting the molar ratio of Na element to metal doped element in the material, improves the multiplying power performance and the circulation stability of the material, and further strengthens the chemical stability and the thermal stability of the material by the in-situ generated conductive carbon layer, thereby endowing the material with certain heat resistance and solvent resistance.
2) According to the preparation method provided by the invention, one-time feeding is adopted, the raw materials are fully and uniformly mixed by high-energy ball milling, and on the other hand, the activation energy of the surface of the material is changed, so that pure-phase compounds are conveniently formed in the subsequent sintering process, and the purity of the material is effectively improved; the method has the advantages of simple process, low cost and the like, and is suitable for industrial production.
3) In the technical scheme provided by the invention, the sodium ion battery prepared based on the composite material provided by the invention has a higher voltage platform, excellent chemical stability and thermal stability, and the phase change process is still stable under a wide electrochemical window.
Drawings
FIG. 1 is an XRD pattern of the composites prepared in examples 1, 2 and 6;
FIG. 2 is a thermogravimetric plot of the composite materials prepared in example 1 and example 6;
FIG. 3 is an XPS diagram of the composite material prepared in example 2;
wherein, fig. 3 (a) is a full spectrum, fig. 3 (b) is a C1 s spectrum, fig. 3 (C) is a Fe 2p spectrum, and fig. 3 (d) is a Ti 2p spectrum;
FIG. 4 is a cyclic voltammogram of examples 1 and 2;
wherein, fig. 4 (a) is a cyclic voltammogram of the material prepared in example 1 assembled into a sodium ion battery, and fig. 4 (b) is a cyclic voltammogram of the material prepared in example 2 assembled into a sodium ion battery;
FIG. 5 is a graph showing the cycling performance at 5C of sodium ion batteries assembled from the materials prepared in examples 1, 2, 6 and comparative example 6;
fig. 6 is a graph showing the rate performance of sodium ion batteries assembled from the materials prepared in examples 1, 2, and 6 and comparative example 6.
Detailed Description
The invention is further illustrated by the following examples, which are not intended to limit the invention.
Example 1
Sodium acetate, ferrous acetylacetonate, phosphoric acid, carbon nanotubes and citric acid are used as raw materials, and ethanol is used as a solvent. Wherein sodium acetate is a sodium source, ferrous acetylacetonate is an iron source, phosphoric acid is a phosphorus source, carbon nanotubes are a carbon source, and citric acid is both a carbon source and a reducing agent;
the mole ratio of Na element, fe element, P element and reducing agent is 3.12:2.44:4:3.5 weighing sodium acetate, ferrous acetylacetonate, phosphoric acid and citric acid, weighing a certain amount of carbon nanotubes, adding the weighed raw materials into a proper amount of absolute ethyl alcohol, ball-milling for 8 hours at the rotating speed of 500r/min, transferring the uniformly mixed materials into a 120 ℃ oven, and vacuum drying to obtain a powdery precursor. Grinding the precursor, tabletting under 10MPa, and placing the flaky precursor in a reducing atmosphere Ar/H 2 (containing 5%H) 2 ) High-temperature sintering at a temperature rising rate of 2 ℃ per minute, a temperature control of 500 ℃ and a heat preservation time of 10 hours to obtain Na 3.12 Fe 2.44 (P 2 O 7 ) 2 And C, the in-situ coating carbon layer accounts for 12% of the total mass of the composite material.
When the button cell (CR 2016) is assembled, the prepared Na is weighed according to the mass ratio of 7:2:1 3.12 Fe 2.44 (P 2 O 7 ) 2 The positive electrode/C, super P and PVDF were ground uniformly in an agate mortar and added with an appropriate amount of NMP solvent to obtain a paste of moderate viscosity, which was coated on aluminum foil and transferred to a vacuum oven at 80℃for more than 6 hours to ensure complete drying. The mass of each electrode slice is about 2mg by adopting a cutter head cutting slice with the diameter of 12 mm. With metallic sodium as counter electrode, with 1.0M NaClO 4 in PC with 5% FEC as electrolyte, the cell assembly process was completed in an inert gas glove box (UNILAB MBRAUND, domestic), and the activation process was performed in an incubator at 30 ℃. The electrochemical data of the battery is tested and collected by a Xinwei battery charge-discharge instrument, and a constant-current charge-discharge mode is adopted, wherein the voltage range is 1.5-4.0V. The cell was tested by means of a Shanghai Chenhua electrochemical workstation at a scanning speed of 0.2 mV/s.
Example 2
The difference compared to example 1 is that the dopant is added, in particular as follows:
sodium acetate, ferrous acetylacetonate, phosphoric acid, carbon nanotubes and citric acid are used as raw materials, tetrabutyl titanate is used as a doping agent, and ethanol is used as a solvent. Wherein sodium acetate is a sodium source, ferrous acetylacetonate is an iron source, phosphoric acid is a phosphorus source, carbon nanotubes are a carbon source, citric acid is both a carbon source and a reducing agent, and tetrabutyl titanate is a titanium source;
the mole ratio of Na element, fe element, ti element, P element and reducing agent is 2.82:2.24:0.2:4:3.5 weighing sodium acetate, ferrous acetylacetonate, tetrabutyl titanate, phosphoric acid and citric acid, weighing a certain amount of carbon nanotubes, adding the weighed raw materials into a proper amount of absolute ethyl alcohol, ball-milling for 8 hours at the rotating speed of 500r/min, transferring the uniformly mixed materials into a 120 ℃ oven, and vacuum drying to obtain a powdery precursor. Grinding the precursor, tabletting under 10MPa, and placing the flaky precursor in a reducing atmosphere Ar/H 2 (containing 5%H) 2 ) High-temperature sintering at a temperature rising rate of 2 ℃ per minute, a temperature control of 500 ℃ and a heat preservation time of 10 hours to obtain Na 2.82 Fe 2.24 Ti 0.2 (P 2 O 7 ) 2 And (C) the in-situ coated carbon layer accounts for 11.8% of the total mass of the composite material.
Preparation of material smears and button cell and electrochemical performance test the same as in example 1.
Example 3
The difference compared to example 1 is that the dopant is added, in particular as follows:
sodium acetate, ferrous acetylacetonate, phosphoric acid, carbon nanotubes and citric acid are used as raw materials, manganese oxalate is used as a doping agent, and ethanol is used as a solvent. Wherein sodium acetate is a sodium source, ferrous acetylacetonate is an iron source, phosphoric acid is a phosphorus source, carbon nanotubes are a carbon source, citric acid is both a carbon source and a reducing agent, and manganese oxalate is a manganese source;
the mole ratio of Na element, fe element, mn element, P element and reducing agent is 3.12:2.24:0.2:4:3.5 weighing sodium acetate, ferrous acetylacetonate, manganese oxalate, phosphoric acid and citric acid, weighing a certain amount of carbon nanotubes, adding the weighed raw materials into a proper amount of absolute ethyl alcohol, ball-milling for 8 hours at the rotating speed of 500r/min, transferring the uniformly mixed materials into a 120 ℃ oven, and vacuum drying to obtain a powdery precursor. Grinding the precursor, tabletting under 10MPa, and placing the flaky precursor in a reducing atmosphere Ar/H 2 (containing 5%H) 2 ) Middle and high gradeSintering at a temperature of 2 ℃ per minute, controlling the temperature to 500 ℃ and keeping the temperature for 10 hours to obtain Na 3.12 Fe 2.24 Mn 0.2 (P 2 O 7 ) 2 And C, the in-situ coated carbon layer accounts for 12.2% of the total mass of the composite material.
Preparation of material smears and button cell and electrochemical performance test the same as in example 1.
Example 4
The difference compared to example 1 is that the sintering temperature is increased, specifically as follows:
sodium acetate, ferrous acetylacetonate, phosphoric acid, carbon nanotubes and citric acid are used as raw materials, and ethanol is used as a solvent. Wherein sodium acetate is a sodium source, ferrous acetylacetonate is an iron source, phosphoric acid is a phosphorus source, carbon nanotubes are a carbon source, and citric acid is both a carbon source and a reducing agent;
the mole ratio of Na element, fe element, P element and reducing agent is 3.12:2.44:4:3.5 weighing sodium acetate, ferrous acetylacetonate, phosphoric acid and citric acid, weighing a certain amount of carbon nanotubes, adding the weighed raw materials into a proper amount of absolute ethyl alcohol, ball-milling for 8 hours at the rotating speed of 500r/min, transferring the uniformly mixed materials into a 120 ℃ oven, and vacuum drying to obtain a powdery precursor. Grinding the precursor, tabletting under 10MPa, and placing the flaky precursor in a reducing atmosphere Ar/H 2 (containing 5%H) 2 ) High-temperature sintering at a temperature rising rate of 2 ℃ per minute, a temperature control of 550 ℃ and a heat preservation time of 10 hours to obtain Na 3.12 Fe 2.44 (P 2 O 7 ) 2 And C, the in-situ coated carbon layer accounts for 12.1% of the total mass of the composite material.
Preparation of material smears and button cell and electrochemical performance test the same as in example 1.
Example 5
Compared with example 1, the difference is that the sintering time is reduced, specifically as follows:
sodium acetate, ferrous acetylacetonate, phosphoric acid, carbon nanotubes and citric acid are used as raw materials, and ethanol is used as a solvent. Wherein sodium acetate is a sodium source, ferrous acetylacetonate is an iron source, phosphoric acid is a phosphorus source, carbon nanotubes are a carbon source, and citric acid is both a carbon source and a reducing agent;
the mole ratio of Na element, fe element, P element and reducing agent is 3.12:2.44:4:3.5 weighing sodium acetate, ferrous acetylacetonate, phosphoric acid and citric acid, weighing a certain amount of carbon nanotubes, adding the weighed raw materials into a proper amount of absolute ethyl alcohol, ball-milling for 8 hours at the rotating speed of 500r/min, transferring the uniformly mixed materials into a 120 ℃ oven, and vacuum drying to obtain a powdery precursor. Grinding the precursor, tabletting under 10MPa, and placing the flaky precursor in a reducing atmosphere Ar/H 2 (containing 5%H) 2 ) High-temperature sintering at a temperature rising rate of 2 ℃ per minute, a temperature control of 500 ℃ and a heat preservation time of 2 hours to obtain Na 3.12 Fe 2.44 (P 2 O 7 ) 2 And (C) the in-situ coated carbon layer accounts for 11.5% of the total mass of the composite material.
Preparation of material smears and button cell and electrochemical performance test the same as in example 1.
Example 6
The difference compared with example 1 is that the addition amount of the carbon source is reduced, specifically as follows:
sodium acetate, ferrous acetylacetonate, phosphoric acid and citric acid are used as raw materials, and ethanol is used as a solvent. Wherein sodium acetate is a sodium source, ferrous acetylacetonate is an iron source, phosphoric acid is a phosphorus source, and citric acid is a carbon source and a reducing agent;
the mole ratio of Na element, fe element, P element and reducing agent is 3.12:2.44:4:3.5 weighing sodium acetate, ferrous acetylacetonate, phosphoric acid and citric acid, adding the weighed raw materials into proper amount of absolute ethyl alcohol, ball-milling for 8 hours at the rotating speed of 500r/min, transferring the uniformly mixed materials into a 120 ℃ oven, and vacuum drying to obtain a powdery precursor. Grinding the precursor, tabletting under 10MPa, and placing the flaky precursor in a reducing atmosphere Ar/H 2 (containing 5%H) 2 ) High-temperature sintering at a temperature rising rate of 2 ℃ per minute, a temperature control of 500 ℃ and a heat preservation time of 10 hours to obtain Na 3.12 Fe 2.44 (P 2 O 7 ) 2 And (C) the in-situ coated carbon layer accounts for 5.81% of the total mass of the composite material.
Preparation of material smears and button cell and electrochemical performance test the same as in example 1.
Comparative example 1
Sodium acetate, ferrous acetylacetonate, phosphoric acid, carbon nanotubes and citric acid are used as raw materials, and ethylene glycol is used as a solvent. Wherein sodium acetate is a sodium source, ferrous acetylacetonate is an iron source, phosphoric acid is a phosphorus source, carbon nanotubes are a carbon source, and citric acid is both a carbon source and a reducing agent;
the mole ratio of Na element, fe element, P element and reducing agent is 3.12:2.44:4:3.5 weighing sodium acetate, ferrous acetylacetonate, phosphoric acid and citric acid, weighing a certain amount of carbon nanotubes, adding the weighed raw materials into ethylene glycol, ball-milling for 8 hours at the rotating speed of 500r/min, transferring the uniformly mixed materials into a 120 ℃ oven, and vacuum drying to obtain a powdery precursor. Grinding the precursor, tabletting under 10MPa, and placing the flaky precursor in a reducing atmosphere Ar/H 2 (containing 5%H) 2 ) High-temperature sintering at a temperature rising rate of 2 ℃ per minute, a temperature control of 500 ℃ and a heat preservation time of 10 hours to obtain Na 3.12 Fe 2.44 (P 2 O 7 ) 2 And C, the in-situ coating carbon layer accounts for 12% of the total mass of the composite material.
Preparation of material smears and button cell and electrochemical performance test the same as in example 1.
Comparative example 2
Sodium metaphosphate, ferrous acetylacetonate, phosphoric acid, carbon nano-tube and citric acid are used as raw materials, and ethanol is used as a solvent. Wherein, sodium metaphosphate is a sodium source, ferrous acetylacetonate is an iron source, supplemental phosphoric acid is a phosphorus source, carbon nanotubes are a carbon source, and citric acid is both a carbon source and a reducing agent;
the mole ratio of Na element, fe element, P element and reducing agent is 3.12:2.44:4:3.5 weighing sodium metaphosphate, ferrous acetylacetonate, phosphoric acid and citric acid, weighing a certain amount of carbon nanotubes, adding the weighed raw materials into a proper amount of absolute ethyl alcohol, ball-milling for 8 hours at the rotating speed of 500r/min, transferring the uniformly mixed materials into a 120 ℃ oven, and vacuum drying to obtain a powdery precursor. Grinding the precursor, tabletting under 10MPa, and placing the precursor in a state of being stillOriginal atmosphere Ar/H 2 (containing 5%H) 2 ) High-temperature sintering at a temperature rising rate of 2 ℃ per minute, a temperature control of 500 ℃ and a heat preservation time of 10 hours to obtain Na 3.12 Fe 2.44 (P 2 O 7 ) 2 And (C) the in-situ coated carbon layer accounts for 11.8% of the total mass of the composite material.
Preparation of material smears and button cell and electrochemical performance test the same as in example 1.
Comparative example 3
Sodium acetate, ferric chloride, phosphoric acid, carbon nano tubes and citric acid are used as raw materials, and ethanol is used as a solvent. Wherein sodium acetate is a sodium source, ferric chloride is an iron source, phosphoric acid is a phosphorus source, carbon nanotubes are a carbon source, and citric acid is both a carbon source and a reducing agent;
the mole ratio of Na element, fe element, P element and reducing agent is 3.12:2.44:4:3.5 weighing sodium acetate, ferric chloride, phosphoric acid and citric acid, weighing a certain amount of carbon nanotubes, adding the weighed raw materials into a proper amount of absolute ethyl alcohol, ball-milling for 8 hours at the rotating speed of 500r/min, transferring the uniformly mixed materials into a 120 ℃ oven, and vacuum drying to obtain a powdery precursor. Grinding the precursor, tabletting under 10MPa, and placing the flaky precursor in a reducing atmosphere Ar/H 2 (containing 5%H) 2 ) High-temperature sintering at a temperature rising rate of 2 ℃ per minute, a temperature control of 500 ℃ and a heat preservation time of 10 hours to obtain Na 3.12 Fe 2.44 (P 2 O 7 ) 2 And (C) the in-situ coated carbon layer accounts for 11.9% of the total mass of the composite material.
Preparation of material smears and button cell and electrochemical performance test the same as in example 1.
Comparative example 4
Sodium acetate, ferrous acetylacetonate, guanidine phosphate, carbon nanotubes and citric acid are used as raw materials, and ethanol is used as a solvent. Wherein sodium acetate is a sodium source, ferrous acetylacetonate is an iron source, guanidine phosphate is a phosphorus source, carbon nanotubes are a carbon source, and citric acid is both a carbon source and a reducing agent;
the mole ratio of Na element, fe element, P element and reducing agent is 3.12:2.44:4:3.5 weighing sodium acetate, ferrous acetylacetonate, phosphorusAnd (3) acid guanidine and citric acid, weighing a certain amount of carbon nanotubes, adding the weighed raw materials into a proper amount of absolute ethyl alcohol, ball-milling for 8 hours at the rotating speed of 500r/min, transferring the uniformly mixed materials into a 120 ℃ oven, and vacuum drying to obtain a powdery precursor. Grinding the precursor, tabletting under 10MPa, and placing the flaky precursor in a reducing atmosphere Ar/H 2 (containing 5%H) 2 ) High-temperature sintering at a temperature rising rate of 2 ℃ per minute, a temperature control of 500 ℃ and a heat preservation time of 10 hours to obtain Na 3.12 Fe 2.44 (P 2 O 7 ) 2 And (C) the in-situ coated carbon layer accounts for 12.3% of the total mass of the composite material.
Preparation of material smears and button cell and electrochemical performance test the same as in example 1.
Comparative example 5
Sodium acetate, ferrous acetylacetonate, phosphoric acid, cellulose and citric acid are used as raw materials, and ethanol is used as a solvent. Wherein sodium acetate is a sodium source, ferrous acetylacetonate is an iron source, phosphoric acid is a phosphorus source, cellulose is a carbon source, and citric acid is both a carbon source and a reducing agent;
the mole ratio of Na element, fe element, P element and reducing agent is 3.12:2.44:4:3.5 weighing sodium acetate, ferrous acetylacetonate, phosphoric acid and citric acid, weighing a certain amount of cellulose, adding the weighed raw materials into a proper amount of absolute ethyl alcohol, ball-milling for 8 hours at the rotating speed of 500r/min, transferring the uniformly mixed materials into a 120 ℃ oven, and vacuum drying to obtain a powdery precursor. Grinding the precursor, tabletting under 10MPa, and placing the flaky precursor in a reducing atmosphere Ar/H 2 (containing 5%H) 2 ) High-temperature sintering at a temperature rising rate of 2 ℃ per minute, a temperature control of 500 ℃ and a heat preservation time of 10 hours to obtain Na 3.12 Fe 2.44 (P 2 O 7 ) 2 And C, the in-situ coated carbon layer accounts for 12.2% of the total mass of the composite material.
Preparation of material smears and button cell and electrochemical performance test the same as in example 1.
Comparative example 6
Sodium acetate, ferrous acetylacetonate, phosphoric acid, carbon nanotubes and citric acid are used as raw materials, and ethanol is used as a solvent. Wherein sodium acetate is a sodium source, ferrous acetylacetonate is an iron source, phosphoric acid is a phosphorus source, carbon nanotubes are a carbon source, and citric acid is both a carbon source and a reducing agent;
the molar ratio of Na element, fe element, P element and reducing agent is 2:1:2:1.5 weighing sodium acetate, ferrous acetylacetonate, phosphoric acid and citric acid, weighing a certain amount of carbon nanotubes, adding the weighed raw materials into a proper amount of absolute ethyl alcohol, ball-milling for 8 hours at the rotating speed of 500r/min, transferring the uniformly mixed materials into a 120 ℃ oven, and vacuum drying to obtain a powdery precursor. Grinding the precursor, tabletting under 10MPa, and placing the flaky precursor in a reducing atmosphere Ar/H 2 (containing 5%H) 2 ) High-temperature sintering at a temperature rising rate of 2 ℃ per minute, a temperature control of 500 ℃ and a heat preservation time of 10 hours to obtain Na 2 FeP 2 O 7 And (C) the in-situ coated carbon layer accounts for 11.5% of the total mass of the composite material.
Preparation of material smears and button cell and electrochemical performance test the same as in example 1.
Table 1 is the electrochemical performance of assembled sodium ion batteries at a current density of 5C using the target materials prepared in examples 1-6 and comparative examples 1-6 as positive electrodes.
According to the carbon-coated pyrophosphoric acid polyanion composite material, a preparation method and application thereof are provided. The metal element M is doped into a unit cell of the pyrophosphoric acid polyanion material to replace part of iron ions, and in-situ carbon is uniformly coated on the surface of the doped pyrophosphoric acid polyanion material, so that the optimized composite material is obtained. As shown in FIG. 1, diffraction peaks of the pyrophosphoric acid polyanion material prepared in the example can be successfully compared with standard cards, which shows that the material is pure phase and has good crystallinity. As can be seen from fig. 3 and 5, the addition of the dopant can successfully incorporate the metal element M and exist in the material in a suitable valence state. Compared with the comparative example, the selection of the solvent, the sodium source, the phosphorus source and other raw materials is particularly important, for example, in the comparative example 3, ferric chloride is adopted as the iron source, and the existence of chlorine element can cause the materialThe generation of impurity phases in the preparation process can not meet the technical requirements. The introduction of M element helps to realize Na + Disordered rearrangement of active sites in Na (2) is carried out, thereby avoiding adverse discharge behavior in a low-voltage area, further improving average working voltage, realizing full Na ion storage in a high-voltage platform area, and the average working voltage of the material prepared in the embodiment 2 is close to 3.0V, and the gram capacity of initial discharge is up to 79mAh g at 5C multiplying power -1 After 500 cycles, the capacity retention was 97.8%. The capacity can reach 58mAh g at 30C high multiplying power -1 . The output voltage, specific discharge capacity, stability and the like are obviously superior to those of other examples and comparative examples. It can be found that the doping of the metal element M strengthens the unit cell structure of the material and improves the stability of the charge-discharge process. The carbon nano tube and the citric acid are calcined to form a carbon layer which is wrapped on the surface of the material in situ, so that the intrinsic electronic conductivity of the material is greatly improved, and the material has high discharge capacity under high multiplying power. In addition, the polyanion and NaFeP provided by the invention 2 O 7 And Na (Na) 2 FeP 2 O 7 Different from equal pyrophosphoric acid polyanion, non-stoichiometric Na 3.12-y Fe 2.44- x M x (P 2 O 7 ) 2 The lattice structure of the material produces defects, and the broad path increases the migration rate of sodium ions.
In conclusion, the carbon-coated pyrophosphoric acid polyanion composite material provided by the invention is prepared by a ball milling method, and has excellent electrochemical performance as a positive electrode of a sodium ion battery.
Claims (9)
1. A preparation method of a carbon-coated pyrophosphoric acid polyanion composite material is characterized by comprising the following steps: dispersing raw materials comprising a sodium source, an iron source, a phosphorus source, a doped metal source, a carbon source and a reducing agent in a solvent, and sequentially performing high-energy ball milling, drying, tabletting and sintering to obtain the composite material; the molar ratio of the sodium element, the iron element and the phosphorus element in the raw materials is 3-3.3: 1.8-2.5: 3.9-4.1; the molar ratio of the metal element in the raw materials to the reducing agent is 2-3:3; the sintering process conditions are as follows: in an inert/reducing atmosphere, the temperature is 400-600 ℃ and the time is 6-20 h; the molar ratio of the sodium element to the doped metal element is 3:0.192 to 0.213; the doped metal element is one of Mn, V, ti, mg, al, cu, Y and Zr;
the iron source is ferrous acetylacetonate; the reducing agent is citric acid;
the high-energy ball milling is wet ball milling, and the ball-material ratio is 5-10:1;
the composite material consists of pyrophosphoric acid polyanion and a carbon layer coated on the surface of the pyrophosphoric acid polyanion in situ; the structural general formula of the pyrophosphoric acid polyanion is Na 3.12-y Fe 2.44-x M x (P 2 O 7 ) 2 Wherein x=0.2, y is more than or equal to 0 and less than or equal to 0.3, and M is a doped metal element.
2. The method for preparing the carbon-coated pyrophosphate polyanion composite material according to claim 1, wherein: the conditions of the high-energy ball milling are as follows: the rotating speed is 300-500 r/min, and the time is 6-24 h.
3. The method for preparing the carbon-coated pyrophosphate polyanion composite material according to claim 1, wherein: the sodium source is one of trisodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium oxide and sodium peroxide;
the doped metal source is at least one of inorganic salt, organic salt, hydroxide and oxide containing doped metal elements; the phosphorus source is one of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, ammonium dihydrogen phosphate and sodium pyrophosphate.
4. The method for preparing the carbon-coated pyrophosphate polyanion composite material according to claim 1, wherein: the carbon source is one of graphene, carbon nanotubes, conductive carbon black, sucrose, oxalic acid, glucose, ascorbic acid, polyvinylpyrrolidone and citric acid.
5. The method for preparing the carbon-coated pyrophosphate polyanion composite material according to claim 1, wherein: the solvent is one of deionized water, ethanol, isopropanol and acetone.
6. The method for preparing the carbon-coated pyrophosphate polyanion composite material according to claim 1, wherein: the drying mode is one of forced air drying, vacuum drying and freeze drying; the pressure in the tabletting process is 4-20 MPa.
7. A carbon-coated pyrophosphate polyanion composite material characterized by: the process according to any one of claims 1 to 6.
8. A carbon-coated pyrophosphate polyanion composite material according to claim 7 wherein: the composite material crystal system is a triclinic system, and the space group is P-1; the in-situ carbon coating accounts for 3-15% of the total mass of the composite material.
9. Use of a carbon-coated pyrophosphate polyanion composite material according to claim 7 or 8, characterized in that: as the positive electrode active material of the sodium ion battery.
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| CN105938904A (en) * | 2016-05-31 | 2016-09-14 | 中南大学 | Composite positive electrode material for sodium-ion battery and preparation method of composite positive electrode material |
| CN106784727A (en) * | 2017-01-17 | 2017-05-31 | 东莞市迈科新能源有限公司 | A kind of polyanionic sodium-ion battery positive material and preparation method thereof |
| CN107978738A (en) * | 2017-11-13 | 2018-05-01 | 中南大学 | A kind of composite positive pole of manganese pyrophosphate sodium/carbon and its preparation and application |
| CN108123129A (en) * | 2018-01-04 | 2018-06-05 | 中南大学 | A kind of carbon coating ferric sodium pyrophosphate material and preparation method thereof and the application as sodium-ion battery positive material |
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| CN105938904A (en) * | 2016-05-31 | 2016-09-14 | 中南大学 | Composite positive electrode material for sodium-ion battery and preparation method of composite positive electrode material |
| CN106784727A (en) * | 2017-01-17 | 2017-05-31 | 东莞市迈科新能源有限公司 | A kind of polyanionic sodium-ion battery positive material and preparation method thereof |
| CN107978738A (en) * | 2017-11-13 | 2018-05-01 | 中南大学 | A kind of composite positive pole of manganese pyrophosphate sodium/carbon and its preparation and application |
| CN108123129A (en) * | 2018-01-04 | 2018-06-05 | 中南大学 | A kind of carbon coating ferric sodium pyrophosphate material and preparation method thereof and the application as sodium-ion battery positive material |
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