CN116487544A

CN116487544A - Coralloid carbon-coated ferrous fluoride, and preparation method and application thereof

Info

Publication number: CN116487544A
Application number: CN202211157637.2A
Authority: CN
Inventors: 张永志; 许玉林; 熊文静; 陈立; 邱雯; 邱志明
Original assignee: Sichuan Manxin New Materials Co ltd; Sichuan University
Current assignee: Sichuan Manxin New Materials Co ltd; Sichuan University
Priority date: 2022-09-22
Filing date: 2022-09-22
Publication date: 2023-07-25

Abstract

The invention discloses coralloid carbon-coated ferrous fluoride, a preparation method and application thereof, wherein the preparation method comprises the following steps: step 1: the prepared light green FeSiF ₆ .6H ₂ Adding O powder and PAN powder into deionized water, stirring, and evaporating to dryness to obtain FeSiF ₆ Mixed powder with PAN; step 2: pouring the mixed powder into a hollow copper pipe for sealing, and then placing the hollow copper pipe into a tube furnace for calcination to obtain the composite material. The invention provides a simple sealed solid phase calcination method, which synthesizes trace oxygen doped carbon coated coral plexiform nano ferrous fluoride (FeF) in situ by one step ₂ @C) achieving a balance between the nanosized particle size and the structural stability of the iron-based fluoride conversion material, wherein polypropyleneThe nitrile (PAN) derived carbon coating shell plays a role of limiting the domain, and inhibits FeF in the process of charging/discharging the battery ₂ Overgrowth after grain aggregation, and simultaneously, trace oxygen is automatically introduced in the closed heat treatment to replace a fluoride ion lattice, so that the electrochemical performance of the electrode is effectively enhanced by the synergistic effect of the oxygen and the fluorine ion lattice.

Description

Coralloid carbon-coated ferrous fluoride, and preparation method and application thereof

Technical Field

The invention relates to the technical field of material preparation, in particular to coralloid carbon-coated ferrous fluoride, and a preparation method and application thereof.

Background

LIBs have been used in more sophisticated technology and markets, but most of the positive electrode materials used in current commercial energy storage batteries and power cells are still de-intercalation materials, such as LiCoO ₂ (LCO)、LiFePO ₄ (LFP) and lithium nickel cobalt manganese oxide (NCM), which are limited by the respective theoretical upper specific capacity limits and the earth's limited cobalt and nickel resources, have been difficult to meet for large-scale applications. Meanwhile, in the exploration of SIBs positive electrode materials, transition metal oxides, polyanion compounds and Prussian blue compounds are widely focused by researchers, but the problems of poor theoretical specific capacity and stability of the materials caused by large ionic radius of sodium ions still prevent the practical application of the SIBs.

Compared with the single-electron reaction embedded positive electrode material, the multi-electron conversion reaction conversion positive electrode material has higher theoretical specific capacity, can avoid using a large amount of noble metals, has the advantages of low price, low toxicity and the like, and therefore gradually enters the line of sight of researchers. The electrode materials which have been used for conversion reaction at present are mainly sulfides, fluorides, metal nitrides, etc., because iron is the fourth most abundant and cheapest metal element in the earth, a typical representative of which is expected to satisfy the mass production use of any kind of battery is iron-based conversion in consideration of the abundance of the element and the actual performancePositive electrode material, i.e. FeF ₃ With FeF ₂ 。FeF ₃ Up to 712mA h g by three-electron conversion ^-1 But compared with FeF ₂ ，FeF ₃ More LiF (3li+fef) is formed after the conversion reaction ₃ 3 LiF+Fe) to reduce FeF ₃ Is finally caused to FeF ₃ The reversible specific capacity after ten cycles tends to be much lower than its theoretical specific capacity. And FeF ₂ 571mA h g by two-electron conversion ^-1 Is far higher than the specific capacity of several currently mainstream commercial de-intercalation cathode materials (e.g., LCO 274mA h g ^-1 ，LPF 170mA h g ^-1 ) At the same time, the average working voltage of 2.66V corresponds to 1519 Wh kg ^-1 Is a high energy density. Unfortunately, iron-based fluorides exhibit strong insulation due to a large band gap, and also face problems of a large Fe/LiF interface energy barrier during charging and side reactions between an active material and an electrolyte during charging/discharging, which seriously impair its cycle performance, thus making it difficult to market.

In order to solve the problems, researchers have adopted strategies such as morphology regulation, doping and compounding of active substances and other methods such as modification design of electrolyte to enhance the performance of the iron-based fluoride battery. For example, feF is obtained by electrospinning ₃ Compounding with Polyacrylonitrile (PAN), and preparing FeF after subsequent carbonization and fluorination ₃ Carbon-composited nanofiber, feF ₃ After combining the flexibility and conductivity of the carbon fiber, the carbon fiber is used as a lithium battery anode to realize the anode at 100mA g ^-1 500mA h g at current density ^-1 Is almost 100% capacity retention after 400 cycles. Xiao et al synthesized approximately 20nm wide monodisperse nano FeF by colloid ₂ The nanorods used as the positive electrode of lithium battery are ionic liquid electrolyte (1M LiFSI/Pyr _1,3 FSI) after 50 cycles at C/20>Capacity retention of 90%.

It is worth mentioning that the conversion material can reach theoretical charge/discharge specific capacity after the particle size is properly reduced due to poor intrinsic conductivity, but in terms of preparation process，FeF ₂ In the heating process>It is difficult to maintain chemical stability at 600 ℃ even if FeF is protected by inert gas ₂ Conversion to iron oxide compounds (e.g. Fe ₃ O ₄ ) At the same time FeF ₂ The grains are also agglomerated and grown, so that it is a considerable matter how to achieve stable structure on the premise of keeping the activity of the material in the preparation process.

Disclosure of Invention

The invention aims to provide a preparation method of coralloid carbon-coated ferrous fluoride, which realizes the balance between the nanometer grain diameter and the structural stability of an iron-based fluoride conversion material, wherein a Polyacrylonitrile (PAN) -derived carbon-coated shell layer plays a role of limiting the domain, and FeF is inhibited in the charging/discharging process of a battery ₂ Overgrowth after grain aggregation, and simultaneously, trace oxygen is automatically introduced in the closed heat treatment to replace a fluoride ion lattice, so that the electrochemical performance of the electrode is effectively enhanced by the synergistic effect of the oxygen and the fluorine ion lattice.

In addition, the invention also discloses coralloid carbon coated ferrous fluoride and application thereof.

In order to solve the technical problems, the invention adopts the following technical scheme:

the invention discloses a preparation method of coralloid carbon-coated ferrous fluoride, which comprises the following steps:

step 1: the prepared light green FeSiF ₆ .6H ₂ Adding O powder and PAN powder into deionized water, magnetically stirring for 35-45min, and evaporating to obtain FeSiF ₆ Mixed powder with PAN;

step 2: pouring the mixed powder into a hollow copper tube, sealing two ends of the copper tube in a glove box, and calcining at 700 ℃ in a tube furnace to obtain FeF ₂ @C。

Wherein FeF ₂ The specific formation process of @ C is as follows,

(1) Fe first dissolved in water ²⁺ Adsorbing on the surface of negatively charged PAN particles under Coulomb force, evaporating to dryness to obtain FeSiF ₆ .6H ₂ O is tightly attached to the surface of PAN, feSiF ₆ .6H ₂ O is a bulk crystal, and PAN is a sphere;

(2) Then in the initial stage of calcination in the closed space, feSiF ₆ .6H ₂ O is firstly removed from the combined water and decomposed into SiF ₄ Gas and FeF ₂ When the temperature is raised to 317 ℃ of the melting point of PAN, the PAN is rapidly melted and starts to absorb the positively charged FeF near the embedding under the action of coulomb force ₂ Crystal grains and cyclization reaction simultaneously;

(3) Finally, after reaching 700 ℃, the PAN is coated with FeF after finishing the pyrolysis process ₂ Particles, while the early evaporated bound water cannot escape again with FeF ₂ Reaction, trace oxygen is introduced into the fluorine ion lattice, and FeF ₂ The particles are fully confined within the PAN pyrolytic carbon, eventually each segment of carbon-coated FeF ₂ The composite fibers are connected with each other to present a porous cross-linked coral shape.

Further defined, in step 1, a pale green FeSiF is prepared ₆ .6H ₂ Powder O and PAN powder according to 8:1 to deionized water.

In the step 1, the magnetic stirring time is 40min.

Wherein, in the step 1, the green FeSiF is light ₆ .6H ₂ The preparation process of the O powder is as follows,

s101: the reduced iron powder and the fluosilicic acid aqueous solution are mixed and stirred for 24 hours at room temperature, and ferrous fluosilicide (FeSiF) is obtained according to the following chemical formula ₆ ·6H ₂ O) precipitation of the aqueous solution with an excess of Fe;

Fe+H ₂ SiF ₆ (acid solution) →FeSiF ₆ ·6H ₂ O (Room temperature)

S102: feSiF prepared by S101 ₆ ·6H ₂ Centrifuging O aqueous solution, separating upper layer pale green clear liquid, evaporating at 110deg.C to dryness to obtain pale green FeSiF ₆ ·6H ₂ O powder.

The invention discloses coralloid carbon-coated ferrous fluoride, which is prepared by the preparation method of the coralloid carbon-coated ferrous fluoride.

The invention discloses an application of coralloid carbon-coated ferrous fluoride, which is prepared by the preparation method of the coralloid carbon-coated ferrous fluoride, and the coralloid carbon-coated ferrous fluoride is used for the positive electrode of a lithium ion battery and a sodium ion battery.

Compared with the prior art, the invention has the following beneficial effects:

the invention adopts a simple sealed solid phase calcination method to synthesize trace oxygen doped carbon coated coral plexiform nano ferrous fluoride (FeF) in situ by one step ₂ @C) realizes the balance between the nano particle size and the structural stability of the iron-based fluoride conversion type material, wherein a Polyacrylonitrile (PAN) derived carbon coating shell layer plays a role of limiting the domain, and FeF is inhibited in the charge/discharge process of the battery ₂ Overgrowth after grain aggregation, and simultaneously, trace oxygen is automatically introduced in the closed heat treatment to replace a fluoride ion lattice, so that the electrochemical performance of the electrode is effectively enhanced by the synergistic effect of the oxygen and the fluorine ion lattice.

The invention uses conductive polymer materials of Polyacrylonitrile (PAN) and ferrous fluosilicate (FeSiF) ₆ .6H ₂ O) is used as a raw material, and carbon-coated nano ferrous fluoride (abbreviated as FeF) with coral cluster morphology is synthesized by a one-step high-temperature solid-phase closed calcination method on the premise of not introducing an additional fluorine source ₂ @C)。

In the heating process, PAN can be in-situ melted and embedded with FeF ₂ Thereby inhibiting grain growth, and deaerated (SiF ₄ ) And forming FeF by self-covering after high temperature (700 ℃) carbonization ₂ PAN-derived conductive carbon layer, nano FeF ₂ The particles are embedded in coral fibers (about 40nm in diameter) while trace amounts of oxygen (possibly from evaporated bound water) are introduced into the fluoride lattice. The inter-crosslinked carbon-based network improves Li during half-cell testing ⁺ /Na ⁺ Diffusion capacity, the "confinement effect" of the derivatized carbon layer effectively inhibits FeF ₂ Overgrowth of grains and improved conductivity of the active material. In short, the present invention has found that FeF can be balanced ₂ Is a key point of nano particle size and stable structure.

The test results show that at 30 ℃, feF ₂ @C as LIBs cathode material at 100mA g ^-1 The current density still remains 484.96mA h g after 50 circles ^-1 Reversible specific capacity, capacity retentionRetention 91.30% at the same time FeF ₂ @C as SIBs cathode material at 20mA g ^-1 The current density is 457.98mA h g in the first discharge ^-1 After 20 cycles, 251mA h g still remained ^-1 Is a specific capacity of (a).

Drawings

FIG. 1 shows coral FeF ₂ Schematic of the preparation and formation process of @ C and FeF ₂ Cross-section of @ C shift reaction.

FIG. 2a is FeF at 10k magnification ₂ SEM image of @ C.

FIG. 2b is FeF at 50k magnification ₂ SEM image of @ C.

FIG. 2c FeF at 100k magnification ₂ SEM image of @ C.

FIG. 2d is FeF ₂ One of the TEM images of @ C.

FIG. 2e is FeF ₂ Second TEM image of @ C.

FIG. 2f is FeF ₂ HR-TEM image of @ C.

FIG. 2g is FeF of the selected region of FIG. 2d ₂ EDX image of @ C.

Fig. 2h shows the distribution of Fe element in the selected region of fig. 2 g.

Fig. 2i is a distribution of F elements of the selected region of fig. 2 g.

Fig. 2j is a distribution of C elements of the selected region of fig. 2 g.

Fig. 2k shows the distribution of N elements in the selected region of fig. 2 g.

FIG. 3a is FeF ₂ Schematic bond formation with PAN derived carbon.

FIG. 3b is FeF ₂ XRD pattern of @ C.

FIG. 3c is FeF ₂ Raman plot of @ C and PAN-700.

Fig. 3d is a high resolution XPS spectrum of Fe.

Fig. 3e is a high resolution XPS spectrum of C.

Fig. 3F is a high resolution XPS spectrum of F.

FIG. 3g is a high resolution XPS spectrum of N.

FIG. 4a is FeF ₂ @C as LIBs positive electrode in 1-4V voltage interval, 100mA g ^-1 At current densityConstant current charge/discharge curve.

FIG. 4b is FeF ₂ @C at 0.1mV s ^-1 CV curve under.

FIG. 4c is FeF ₂ At a range of current densities (0.05-1A g ^-1 ) Is provided.

FIG. 4d is FeF ₂ @C and Nano-FeF ₂ ，Micro-FeF ₂ At 100mA g ^-1 Cycle performance at current density versus graph and coulombic efficiency.

FIG. 4e is FeF ₂ @C，Nano-FeF ₂ With Micro-FeF ₂ At a range of current densities (0.05-3A g ^-1 ) Is a discharge capacity of (a).

FIG. 4f is FeF ₂ @C as SIBs positive electrode in voltage range of 1-4.2V, 20mA g ^-1 Constant current charge/discharge curve at current density.

FIG. 4g is Nano-FeF ₂ As SIBs positive electrode, 20mA g was measured in a voltage range of 1-4.2V ^-1 Constant current charge/discharge curve at current density.

FIG. 4h is Micro-FeF ₂ As SIBs positive electrode, 20mA g was measured in a voltage range of 1-4.2V ^-1 Constant current charge/discharge curve at current density.

FIG. 5a is FeF ₂ Log (i) and Log (v) plots at C and the fitted redox peak b values.

FIG. 5b is FeF ₂ @C at 0.2mV s ^-1 Estimated pseudocapacitance contribution at scan speed.

FIG. 5c is FeF ₂ And (3) a morphology change graph of the positive electrode at the@C after different times of circulation.

Fig. 5d is a graph of the percentage of capacitance and diffusion control capacity contribution at different scan rates.

FIG. 5e is FeF ₂ Three-dimensional energy nyquist plot of @ C after different charge/discharge times.

FIG. 5f is Nano-FeF ₂ Three-dimensional energy nyquist plots after different charge/discharge times.

FIG. 5g is Micro-FeF ₂ Three-dimensional energy nyquist plots after different charge/discharge times.

FIG. 5h shows FeF after 15 charge/discharge cycles ₂ @C and Nano-FeF ₂ EDS spectra of separator in half cell.

FIG. 5i is FeF ₂ Cyclic voltammograms at different scan rates @ C.

FIG. 5j is I after three sets of samples were fitted _p -v ^1/2 Slope curve.

FIG. 6a is FeF ₂ Schematic diagrams of two full-cell configuration schemes of a PGO negative electrode and a LLi negative electrode matched with a C positive electrode.

Fig. 6b is a side view of a pressed PGO negative pole piece.

Fig. 6c is a graph of elemental distribution of a pressed PGO negative electrode plate.

FIG. 6d shows LLi/FeF ₂ Constant current charge/discharge curve for @ C full cell.

Fig. 6e is a constant current charge/discharge curve of a PGO/fef2@c full cell.

FIG. 6f is a graph of the cycling performance of LLi/FeF2@C full cells and PGO/FeF2@C full cells.

Fig. 7 is a diagram showing the state of the FeSiF6.6H2O closely attached to the surface of PAN after being evaporated to dryness.

Fig. 8 is an electron microscope image of PAN-700 containing macroporous carbon microspheres.

Fig. 9 is a Zeta potential map.

FIG. 10 is an X-ray diffraction (XRD) pattern of Nano-FeF2 and Micro-FeF 2.

FIG. 11 is a FeF ₂ And (5) finishing results.

Fig. 12 is a nitrogen adsorption/desorption isotherm plot.

FIG. 13 is a total spectrum of FeF2@C XPS.

FIG. 14 is a thermogram of FeF2 in FeF2@C per unit mass.

FIG. 15 shows the Nano-FeF2 and Micro-FeF2CV curves of lithium batteries.

FIG. 16 is a selected EDS area electron microscope image.

Fig. 17 is a constant current charge-discharge graph of a lithium ion battery.

FIG. 18PAN-700 sodium ion battery constant current charge-discharge curve

Fig. 19 is a fef2@c positive CV test chart.

FIG. 20a is a CV test chart of Nano-FeF 2.

FIG. 20b is a CV test chart of Micro-FeF 2.

FIG. 21 is a drawing of a Nano-FeF2 pole piece electron microscope after different cycles.

Fig. 22 is an equivalent circuit diagram after calculating electrochemical impedance parameters of the electrode, the intercept of the semicircle with the Z-axis in the high frequency region.

Fig. 23 is an electrochemical characteristic diagram of the positive electrode after a series of cycles.

Detailed Description

Example 1

The embodiment discloses a preparation method of coralloid carbon-coated ferrous fluoride, which comprises the following steps:

Wherein FeF ₂ The specific formation process of @ C is as follows,

(3) Finally, after reaching 700 ℃, the PAN is coated with FeF after finishing the pyrolysis process ₂ Particles, while the early evaporated bound water cannot escape again with FeF ₂ Reaction, trace oxygen is introduced into the fluorine ion lattice, and FeF ₂ The particles are completely confined to PAN pyrolysisIn carbon, finally, each section of FeF coated with carbon ₂ The composite fibers are connected with each other to present a porous cross-linked coral shape.

In the step 1, the magnetic stirring time is 40min.

In this example, the pale green FeSiF in step 1 ₆ .6H ₂ The preparation process of the O powder is as follows,

Fe+H ₂ SiF ₆ (acid solution) →FeSiF ₆ ·6H ₂ O (Room temperature)

In addition, the coralloid carbon-coated ferrous fluoride prepared in this example was used for the positive electrode of lithium ion batteries and sodium ion batteries.

In order to facilitate a further understanding of the present invention by those skilled in the art, the present invention is further described below in connection with specific preparation examples.

(1) 2.4g of reduced iron powder (CAS 7439-89-6) and 15.34g of aqueous fluosilicic acid solution (H ₂ SiF ₆ CAS 16961-83-4) was mixed and stirred at room temperature for 24 hours to obtain ferrous fluorosilicate (FeSiF) according to the following formula ₆ ·6H ₂ O) precipitation of the aqueous solution with an excess of Fe;

Fe+H ₂ SiF ₆ (acid solution) →FeSiF ₆ ·6H ₂ O (Room temperature)

(2)FeSiF ₆ ·6H ₂ Centrifuging O aqueous solution, separating upper layer pale green clear liquid, evaporating at 110deg.C to dryness to obtain pale green FeSiF ₆ ·6H ₂ O powder;

(3) 2.4g of pale green FeSiF ₆ ·6H ₂ O powder with 0.3g of Polyacrylonitrile (PAN) according to a weight ratio of 1: mixing 8 mass ratio in 20mL deionized water, stirring for 1h, and evaporating to dryness at 100 ℃;

(4) Packaging 1g of the evaporated mixed powder into a hollow copper pipe with the length of 20cm and the diameter of 1.5cm in a glove box, heating to 700 ℃ at 1 ℃/min under the protection of argon by using a tubular furnace, preserving heat for one hour, and naturally cooling to obtain the c-FeF ₂ @NC(FeF ₂ At the position of>Unstable and easily decomposed at 400 ℃ and coarsened grains, so that it is difficult to prepare FeF at high temperature by conventional methods ₂ The nano material of the composite carbon of (2) often requires the additional introduction of fluorine sources such as HF, NF ₃ ；

(5) 1g of pale green FeSiF ₆ ·6H ₂ Packaging O powder into hollow copper pipe with length of 20cm and diameter of 1.5cm, heating to 700 deg.C at 1 deg.C/min under argon protection with tubular furnace, maintaining for one hr, and naturally cooling to obtain FeF ₂ -700；

(6) 1g of pale green FeSiF ₆ ·6H ₂ Heating O powder to 200 ℃ at a speed of 4 ℃/min under the protection of argon by using a tube furnace, preserving heat for two hours, and naturally cooling to obtain FeF ₂ -200；

(7) Packaging 1g of PAN powder into a hollow copper pipe with the length of 20cm and the diameter of 1.5cm, heating to 700 ℃ at a speed of 1 ℃/min under the protection of argon by using a tubular furnace, preserving heat for one hour, and naturally cooling to obtain PAN-700.

As shown in fig. 1a, fig. 1a shows PAN-derived carbon-coated nano ferrous fluoride (FeF) with coral morphology ₂ Schematic synthesis of @ C).

First, the prepared light green FeSiF ₆ .6H ₂ Powder O and PAN powder according to 8:1 into deionized water, magnetically stirring for 40min, and evaporating to obtain FeSiF ₆ Mixed powder with PAN. Next, to prevent FeF ₂ The mixed powder is poured into a hollow copper pipe at first, then both ends of the copper pipe are sealed in a glove box, and then the copper pipe is placed in a tube furnace for calcination at 700 ℃ to prepare FeF ₂ @C。

The formation process of fef2@c can be described as:

(1) Fe2+ first dissolved in water adsorbs to the negatively charged PAN particle surfaces under coulomb force (fig. 9);

FeSiF after evaporating to dryness ₆ .6H ₂ O is tightly adhered to the PAN surface (FIG. 7, feSiF ₆ .6H ₂ O is a bulk crystal, PAN is a sphere);

in FIG. 7, (a, b) FeSiF ₆ .6H ₂ O-crystal (c, d) PAN particles (e, f) FeSiF ₆ .6H ₂ Coating of the PAN particles with O crystals

(2) Then in the initial stage of calcination in the closed space, feSiF ₆ .6H ₂ O is firstly removed from the combined water and decomposed into SiF ₄ Gas and FeF ₂ When the temperature is raised to the melting point of PAN (317 ℃ C.), PAN rapidly melts and begins to absorb the positively charged FeF near the embedding under Coulomb force (FIG. 9) ₂ Crystal grains and cyclization reaction simultaneously;

Typically, by reducing FeF ₂ The chemical specific surface area can be increased by increasing the particle size, so that the charge/discharge performance of the electrode material can be effectively improved, but the structural stability of the active material can be weakened, the agglomeration phenomenon and adverse reaction between the positive electrode and the electrolyte are aggravated, and finally the stability of the battery is poor.

Therefore, compared with the simple particle size reduction, the FeF after compounding ₂ In @ C, as in fig. 1 b; nano-sized FeF ₂ The particles are embedded in a PAN-derived carbon matrix which, on the one hand, is effective to enhance the structural toughness of the active material and to improve conductivity, allowing the discharge process to occur:

(3Li+FeF ₂ →2LiF+Fe)

the Fe and LiF generated are limited in the cavity of the carbon fiber, thereby avoiding the generation of agglomerated Fe clusters and LiF clusters, on the other hand PPyridine N after AN thermal cracking is beneficial to Li ⁺ /Na ⁺ Transmitting at the same time FeF ₂ Both the network structure and pore channels possessed by @ C facilitate electrolyte wetting in the electrode material.

As a control, feSiF alone ₆ .6H ₂ O powder and PAN alone were also calcined at 700℃after being sealed in copper tubes, respectively, in the same manner (abbreviated as Micro-FeF ₂ With PAN-700), furthermore with FeSiF at 200 DEG C ₆ Powder preparation of nano FeF ₂ (abbreviated as Nano-FeF ₂ )。

FIGS. 2 (a-k) show FeF ₂ Morphology and elemental distribution of @ C samples.

First, feF can be seen by Scanning Electron Microscope (SEM) images (FIGS. 2 a-c) ₂ The @ C sample was coated with FeF by carbon having a diameter of about 40nm ₂ The nano composite fiber is formed, the fiber presents a cross-linking shape, and a large number of pore channels are formed, thus the FeF is formed ₂ The whole at C presents a coral-like morphology.

The composite fiber then appears to exhibit a core-shell structure as seen by Transmission Electron Microscopy (TEM) images (FIGS. 2d, e), with a layer of derivatized carbon overlying the FeF ₂ The nanoparticle surface, high resolution TEM image (FIG. 2 f) shows about 0.27nm and 0.165nm interplanar spacings correspond to FeF ₂ The (101) and (002) faces of the crystal. Finally, fe, F, C, N elements in the energy dispersive spectroscopy (EDX) image (fig. 2 g-k) are uniformly distributed, and the composite fiber is proved to be formed by coating FeF2 by PAN-derived carbon.

In addition, PAN-700 is a carbon microsphere containing many macropores (FIGS. 8a, b);

Micro-FeF prepared at 700 DEG C ₂ Dense particles of micron size (fig. 8c, d);

Nano-FeF prepared at 200 DEG C ₂ In the case of nanoparticles with a particle size of about 8-22nm (fig. 8e, f), this confirms the conclusion that during high temperature calcination, individual nano FeF2 particles will grow into dense micro FeF2 particles, but by introducing thermoplastic PAN, nano FeF2 particles will be embedded in situ in the carbon matrix, the carbon layer between the particles plays a role of "confinement" and thus alleviates nano FeF at high temperature ₂ Agglomeration growth of particles。

In FIG. 8, (a-b) PAN-700, (c-d) Micro-FeF ₂ ,(e-f)Nano-FeF ₂

FIGS. 3 (a-g) show FeF ₂ Structure and bonding characteristics of the @ C sample.

First, FIG. 3a shows the process in FeF ₂ PAN derived carbon and FeF in @ C ₂ The conductive carbon layers doped with nitrogen can improve the conductivity/ion capacity of the material by being tightly bound together by chemical bonds.

Furthermore, raman spectroscopy analyzed FeF ₂ Carbon properties in @ C and PAN-700, FIG. 3C shows the D band (about 1361cm ^-1 ) With G band (about 1582 cm) ^-1 ) Can find FeF ₂ Intensity ratio of D band to G band in @ C (I _D /I _G ) Above PAN-700, which is probably due to Fe catalysis, feF ₂ The degree of graphitization of the carbon in @ C is higher and thus FeF ₂ The @ C possesses faster conductivity.

Next, feF ₂ @C (FIG. 3 b), nano-FeF ₂ With Micro-FeF ₂ (see FIG. 10 c) X-ray diffraction (XRD) patterns showing FeSiF ₆ .6H ₂ O has been completely converted into FeF ₂ (JCPDS No. 45-1062) and no significant impurity phase appears.

FIG. 10 is an XRD pattern in which (a) FeSiF ₆ .6H ₂ O,(b)PAN 700,(c)Nano-FeF ₂ And Micro-FeF ₂

FeF can be found by XRD pattern comparison ₂ @C and Micro-FeF ₂ FeF after high-temperature sealing calcination ₂ The characteristic diffraction peak intensity of (c) becomes large and is shifted to a high angle, which means that part of oxygen in the crystal water which cannot escape replaces fluorine ions, and the crystal crystallinity becomes large. A refinement of Rietveld was obtained using GSAS II software (see FIG. 11, FIG. 11 is FeF ₂ Finishing results @ C), finishing results indicated that after high temperature calcination, 0.013wt% FeOF phase was included.

The Shewler's formula calculation shows that Micro-FeF ₂ Particle size of [ (]>100 nm) far greater than FeF ₂ @C (55.4 nm) and Nano-FeF ₂ (34.0 m), which illustrates PAN melt coated FeF ₂ Can effectively inhibit F at high temperatureeF ₂ And (5) grain growth. Nitrogen adsorption/desorption isotherms (FIG. 12) for the three groups of samples also show Nano-FeF ₂ And FeF ₂ Specific surface area @ C is 61.181cm respectively ^-2 g ^-1 And 27.214cm ^-2 g ^-1 And Micro-FeF ₂ Specific surface area is only 3.960cm ^-2 g ^-1 This also confirms the uncoated FeF ₂ Nanoparticles grow secondarily at high temperature, and thus have a smaller chemical specific surface area.

Finally, X-ray photoelectron Spectroscopy (XPS) characterizes FeF ₂ The chemical nature of @ C, the overall spectrum (FIG. 13) shows FeF ₂ The @ C is mainly composed of C, N, fe, F element.

The peak in the fine spectrum of Fe 2p (FIG. 3 d) at 711.6eV indicates that Fe may comprise O-Fe ³⁺ -F and Fe ²⁺ The peak of 725.0eV for both valence states of F is derived from Fe ²⁺ 2p of (2) _1/2 . The C1 s fine spectrum (FIG. 3 e) shows that the major peak at 284.8eV,286.2eV is attributed to the C-C, C-N, and the minor peak at 289.3eV is attributed to C-F of the PAN-derived carbon matrix, proving FeF ₂ And has bonding effect with PAN derived carbon. The main peak in the fine spectrum of F1 s (FIG. 3F) at 684.9eV is attributed to Fe ²⁺ F, C-F peak at 686.2eV again corroborates PAN-derived carbon matrix with FeF ₂ Intimate contact between them. Peaks in the N1 s fine spectrum (FIG. 3 g) at 398.9eV,400.1eV and 401.8eV are attributed to PAN pyrolyzed pyridine, pyrrole and graphite nitrogen, and in addition, fe and F in XPS account for 33.62at% and 33.57at%, respectively, calculated in half-stoichiometric relation to FeF ₂ The FeOF ratio of the liquid crystal was 0.025wt%, which is close to the finishing result.

XRD, BET and XPS results confirm that calcination in a closed environment introduces trace amounts of oxygen, producing FeOF, while the "finite field" effect of PAN-derived carbon, feF ₂ The @ C achieves a balance between structural stability and nano-particle size.

As in fig. 4a-h, the FeSiF are individually run in nitrogen prior to half cell testing ₆ .6H ₂ O and PAN were subjected to Thermogravimetric (TG) tests to estimate FeF per unit mass ₂ FeF in @ C ₂ The content of (C) was about 81 wt.% (FIG. 14), and we took 1g FeF ₂ Calcining @ C in air, the residual mass (Fe ₂ O ₃ ) 0.7369g, converted FeF ₂ About 86.58 wt.%, both results being substantially identical, calculated as the latter.

Followed by FeF ₂ @C、Nano-FeF ₂ 、Micro-FeF ₂ And PAN-700 were used as LIBs and SIBs positive electrodes, respectively, and then subjected to electrochemical testing at 30 ℃.

FIG. 4a shows FeF ₂ @C as LIBs cathode material at 1.0-4.0V (vs Li/Li) ⁺ ) 100mA g in voltage interval ^-1 Charge/discharge curve of the first three cycles at current, first discharge capacity 531.12mA h g ^-1 This is close to FeF ₂ But a significant voltage drop occurs on the first discharge to 1.68V, the overpotential being due to disproportionation of the converted iron and pseudo intercalation of lithium, whereas the plateau potential rise after the first discharge may be a regenerated FeF ₂ The particle size reduction is caused by the improvement of reaction kinetics, and furthermore FeF ₂ First charge capacity 463.62mA h g @ C ^-1 Wherein a capacity loss of 19.5% may result from a reaction of irreversible side during the first discharge and the generation of a Solid Electrolyte Interface (SEI) film, an increase in discharge capacity in the first ten cycles may be related to the activation of an active material and a solvent.

At the same time, through 0.1mV s ^-1 The first three cycles of Cyclic Voltammetry (CV) test can be found in FeF ₂ In @ C (FIG. 4 b) two pairs of redox peaks (about 3.0/3.3 and 2.0/2.9V) appear, corresponding to FeF, respectively ₂ The intercalation and conversion reactions of (a) while the peak position and current hardly change much.

In contrast, micro-FeF ₂ (FIG. 15 b) the conversion peak current was an order of magnitude lower, while Nano-FeF ₂ The shift reaction peak of (FIG. 15 a) decays rapidly in the first three cycles while Micro-FeF ₂ About 1.25V, which is significantly higher than FeF ₂ 0.97V at C with Nano-FeF ₂ Is 0.93V.

In FIG. 15, CV curve (a) Nano-FeF of lithium battery ₂ And (b) Micro-FeF ₂ .

This result demonstrates FeF ₂ @C has the most stable electrochemical properties and is optimal for storing Li ⁺ Power toAnd lowest electrochemical polarization. In addition, FIG. 4f shows FeF ₂ At 20mA g with @ C as SIBs positive electrode material ^-1 Charge/discharge curve of the first three cycles at current, first discharge capacity 432.01mA h g ^-1 (theory 571mA h g) ^-1 ) 290.98mA h g still remained after 15 circles ^-1 Reversible specific capacity.

Meanwhile, in the control group, nano-FeF ₂ As LIBs (FIG. 17 a) and SIBs positive electrode (FIG. 4 g), 640.00mA h g are respectively given, although the specific capacity for initial discharge is highest ^-1 And 497.36mA h g ^-1 But it decays rapidly leaving only 181.76mA h g after 50 and 15 cycles respectively ^-1 And 35.59mA h g ^-1 And the characteristic of the conversion reaction platform at 2.1-2.2V completely disappears. In contrast to the former, micro-FeF ₂ The specific capacities of the positive electrodes respectively as LIBs (fig. 17 b) and SIBs (fig. 4 h) are poor, while the capacity contributed by PAN is negligible (fig. 17c and fig. 12).

In FIG. 17, (a) is Nano-FeF ₂ Constant current charge-discharge curve, (b) is Micro-FeF ₂ Constant current charge-discharge curve, (c) is PAN-700 constant current charge-discharge curve.

Fig. 18 is a constant current charge-discharge curve of a PAN-700 sodium ion battery.

The difference is caused by the fact that the intrinsic conductivity of fluoride is poor, so that the battery performance is seriously dependent on the morphology of electrode materials, and FeF is carried out in a high-temperature environment ₂ The nano particles are agglomerated and secondarily grown into compact FeF ₂ Microparticles, which severely reduce FeF ₂ Electrochemically active, thus Micro-FeF ₂ The capacity is the lowest. Although Nano-FeF ₂ The specific capacity of the first discharge is highest and even exceeds theoretical values (due to SEI generation and electrode side reaction), but problems such as transition metal dissolution and structural failure which occur in the prior art are more serious in the exposed nano particles. In contrast, in FeF ₂ In @ C, because on the one hand the PAN-derived conductive carbon matrix is specific to nano FeF ₂ The dissolution and agglomeration of transition metals are inhibited by the limiting action of the particles, and the active material structure is reinforced;

on the other hand, coral plexiform nanostructures retain FeF ₂ Both achieve a balance between nano-design and structural stability, and in addition, the introduction of trace amounts of oxygen also improves FeF ₂ Is a semiconductor device, is an intrinsic conductivity of the semiconductor device.

Thus FeF ₂ The @ C positive electrodes of LIBs and SIBs, respectively, exhibited high specific capacities and retention rates (FIGS. 4d and 4 f), feF ₂ The @ C was used as LIBs positive electrode at 100mA g ^-1 After 50 circles of current density, 484.96mA h g still exists ^-1 The reversible capacity of (2) and the obvious conversion reaction platform characteristics, the capacity retention rate is 91.31%. At the same time, also has a stable charge/discharge curve at different currents (FIG. 4 c), at 0.05,0.10,0.20,0.30,0.40,0.50,1.00,2.00 and 3.00A g ^-1 Has current densities of 528.27, 478.73, 437.96, 410.12, 387.12, 364.80, 318.51, 264.47 and 223.65mA h g, respectively ^-1 And (4 e) exhibits excellent rate performance.

In order to explore the mechanism of electrode performance enhancement, the invention adopts Cyclic Voltammetry (CV), electrochemical Impedance Spectroscopy (EIS), energy Dispersive Spectroscopy (EDS) and SEM to carry out comparison analysis on electrochemical, element and morphological characteristics of three groups of samples. The intrinsic mechanism of charge storage of battery materials is a limited diffusion control process that is generally thought to involve surface control and diffusion control.

Therefore, in order to calculate the Li storage of the cathode material ⁺ The constant speed step in the process firstly uses 0.2-1mV s ^-1 Sweep rate to FeF ₂ The @ C positive electrode was subjected to CV testing (FIG. 19).

As the sweep speed changes, it can be clearly found that the peak curve of the CV shows similar shape and position, which reveals good and stable electrochemical kinetics of the material. Wherein the relation between the current (i) and the scan rate (v) satisfies the following formula:

i＝av ^b (1)

log(i)＝log(a)+blog(v) (2)

at different scan speeds, the values of parameters a and b are fitted according to the linear relationship of log (i) to log (v) in the formula. b=1 indicates that the process is dominated by surface control, capacity is derived from capacitive contribution, b=0.5 indicates that the process is dominated by bulk diffusion control, and if b has a value between 0.5 and 1 indicates that the process is controlled by a mixture of surface control and diffusion control. Then calculated, the B values of the a and B peaks were 0.86 and 0.83, respectively (fig. 5 a), indicating that the capacity contribution of the process is of mixed control and dominated by surface control. The duty cycle quantification of the final surface and diffusion controlled current contribution can be calculated using the following formula:

i＝k ₁ v+k ₂ v ^1/2 (3)

as shown in FIG. 5c, at 0.2mV s ^-1 The lower surface reaction controlled capacitance contribution ratio exceeded 65% and the diffusion control degree was reduced with increasing sweep speed, the capacitance/diffusion ratio was progressively greater at 1.0mV s ^-1 The lower limit reaches 84 percent. The evidence fully shows that the pseudocapacitance process of the electrode is surface reaction dominant, and the capacity is more due to the rapid oxidation-reduction reaction of near-surface active sites, so that the FeF subjected to nano-composite modification ₂ The @ C positive electrode exhibited excellent rate performance.

Next, 0.2-1mV s is used ^-1 Is tested for Li under the same assembly conditions and battery conditions for three groups of samples ⁺ The diffusion capacity (fig. 5d and 20a, 1014B) is first determined by the peak currents (I _p )

Square root of the sweep rate (v ^1/2 ) The relationship (FIG. 5 e) can find I _p And v ^1/2 Exhibits a linear correlation therebetween, indicating FeF ₂ Depending on the diffusion control step, followed by the randes-Sevcik equation as follows:

I _p ＝2.69×10 ⁵ n ^3/2 ACD1 ^/2 v ^1/2

wherein n represents the electron transfer number, A represents the effective area (cm) of the working electrode ² ) C represents the concentration of ions (mol cm) involved in the redox reaction ³ ) D is the diffusion coefficient (cm) ² s ^-1 ) Regarding n, A, C as constants, bringing different v ^1/2 And I _p Then calculate Li by fitting the slope of the function ⁺ Apparent diffusion coefficient (Na ⁺ The diffusion coefficient is calculated by the same method, and the calculation result shows thatLi-intercalation ⁺ /Na ⁺ In the conversion reaction of the process, nano-FeF ₂ 、FeF ₂ @C and MiCro-FeF ₂ Li of (2) ⁺ /Na ⁺ The diffusion capacity is sequentially from high to low, but Li is removed ⁺ /Na ⁺ In the conversion reaction of the process, feF ₂ Li at C ⁺ /Na ⁺ The highest diffusion capacity, therefore, feF is taken together ₂ The @ C kinetics was the best. The results confirm the previous conclusion, feF ₂ The high crystallinity and conductive carbon coating in @ C enhance the stability of the nanostructure, and the unique coral crosslinked structure and pyrrole N accelerate Li ⁺ /Na ⁺ The ability to diffuse in both the solid and liquid phases, both of which facilitate rapid charge/discharge processes.

Finally, the EIS tested the electrochemical properties of three groups of samples as LIBs (fig. 5 g-i) and SIBs (fig. 23) positive electrode, respectively, after a series of cycles.

The EIS plot after various cycles in fig. 23, decibels, is: (a) FeF (FeF) ₂ @C，(b)Micro-FeF ₂ ，(b)Nano-FeF ₂ ；

First, by simple comparison, it can be found that compared with FeF ₂ @C，Micro-FeF ₂ And Nano-FeF ₂ As LIBs and SIBs positive electrodes, a significant difference appears in semicircle in the high frequency region, which indicates that the interface and bulk properties of the electrodes are greatly changed. The electrochemical impedance parameters of the three sets of electrodes were then calculated using a simplified equivalent circuit (fig. 22), the intercept of the semicircle to the Z-axis in the high frequency region,

symbol R for representing resistance related to current collector, electrolyte and the like _s Representing the charge transfer resistance and the surface layer (SEI) resistance associated with the electrode/electrolyte bilayer at mid-high frequency by the symbols R, respectively _ct And R is R _CEI As can be seen from the comparison of the annex Table 1, micro-FeF is shown ₂ R after cycling as positive electrode of two batteries _CEI And R is R _ct A large increase occurs with R _ct As the number of cycles increases, this may be due to the continuous agglomeration of grains, accumulation of byproducts, and the like. And Nano-FeF ₂ The same rule is also true when LIBs are used as positive electrodes, but R is the same as SIBs _CEI And R is R _ct The fluctuation of increasing and decreasing occurs at the same time in Nano-FeF ₂ The nano FeF can also be found in the topography map (FIG. 21) after different cycle times of the electrode ₂ The particles showed significant agglomeration and even "overgrowth" after 15 cycles was large dense particles, which just reflects the instability of the bare nanostructures.

In contrast to the former two, feF ₂ @C as LIBs or SIBs positive electrode, R _ct And R is R _CEI Is much smaller in value than the first two groups of samples, and no large fluctuation occurs, which confirms that the coating of the high-conductivity PAN-derived carbon and the introduction of trace oxygen effectively improve the electron transport capacity of the electrode.

While FIG. 5j shows FeF ₂ The positive electrode @ C well maintained coral-like morphology after multiple cycles, and elemental analysis of FIG. 5f also showed FeF on the side of the separator within the cell near the metallic lithium (EDS region selected in FIG. 16) ₂ The Fe signal of @ C is much weaker than that of Nano-FeF ₂ These results demonstrate that during charge and discharge, the "confinement" effect of the carbon matrix effectively inhibits the aggregation, "overgrowth" of grains and dissolution of transition metals, reinforces coral-like nanostructures, and enables FeF to be ₂ The @ C exhibits stable electrochemical kinetics.

To verify FeF ₂ Practical applicability of @ C as shown in fig. 6a, a quantitative (capacity excess of 64%) lithium negative electrode (LLi, fig. 6 b) pressed from titanium foil and pre-lithiated reduced graphite oxide (PGO) after ten cycles in half cell were used as full cell negative electrodes, respectively. Based on the mass of the positive electrode active material, FIG. 6d shows that the voltage is in the range of 1.0-4.0V and 0.10A g ^-1 LLi/FeF at current density ₂ First discharge specific energy 976.84Wh kg of @ C full battery ^-1 This is much higher than LiCoO ₂ 550Wh kg of positive electrode ^-1 。

While FIG. 6e shows a voltage in the range of 0.8-3.0V and 0.02A g ^-1 PGO/FeF at current density ₂ First discharge specific energy 623.06Wh kg of @ C full battery ^-1 . After 20 cycles (FIG. 6 f), LLi/FeF ₂ The capacity retention after the first time of the @ C full cell was 91.65%, which illustrates LLi versus FeF ₂ Good @ CGood compatibility, but at the same time PGO/FeF ₂ The @ C full cell decays significantly during the first ten cycles, possibly due to irreversible lithium loss induced by the graphite-like PGO negative electrode incompatible electrolyte.

The invention designs a high-temperature solid-phase closed calcination method, which can achieve two purposes on the premise of not introducing extra fluorine source and utilizes PAN and FeF ₂ The coulomb attraction of (2) leads the polymer material PAN to be in-situ hot melt embedded with FeF at high temperature ₂ Nanoparticles, feF in a closed system ₂ Trace oxygen is self-introduced to prepare FeF ₂ @C。

For FeF ₂ The problems of poor intrinsic conductivity, phase separation and structural failure in the charge-discharge process, conductive carbon coating and the introduction of trace oxygen bring several advantages.

First, both improve FeF from intrinsic and bulk phases ₂ Conductivity. Next, the carbon coating reinforces the FeF ₂ Unique coral plexiform structure of @ C, and carbon matrix for nano FeF ₂ The "confinement effect" of the particles inhibits Fe dissolution from two-phase separation between Fe and LiF.

Second, the reversibility of the intercalation-switching reaction is improved after the fluorine is replaced by trace oxygen. Thus FeF ₂ The @ C maintains the high specific capacity and simultaneously considers the stability of the microstructure and the electrochemical property, and realizes the balance between the nano particle size and the structural stability. Thanks to the above characteristics, feF ₂ The @ C exhibits high specific discharge capacity, stability and rate capability.

In LIBs, feF ₂ @C at 100mA g ^-1 At current density, the specific capacity of initial discharge is 531.12mAh g ^-1 After 50 circles, 484.96mAh g still exists ^-1 Reversible specific capacity, capacity retention up to 91.30%, and 3Ag ^-1 242.50mAh g still remained at current density ^-1 Reversible capacity.

Simultaneous FeF ₂ First discharge capacity 457.98mAh g of SIBs as @ C ^-1 After 15 circles, 251mAh g still exists ^-1 . In addition, LLi/FeF ₂ All cells @ C and PGO/FeF ₂ 976.84Wh kg are exhibited by the @ C full cell respectively ^-1 And 623.06Wh kg ^-1 Reversible specific energy, validating FeF ₂ Feasibility of practical application.

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. It is therefore intended that the following claims be interpreted as including the preferred embodiments and all such alterations and modifications as fall within the scope of the invention.

The foregoing description of the preferred embodiment of the invention is not intended to be limiting, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and principles of the invention.

Claims

1. The preparation method of coralloid carbon coated ferrous fluoride is characterized by comprising the following steps:

2. A method for preparing coral-shaped carbon-coated ferrous fluoride as defined in claim 1, wherein: feF (FeF) ₂ The specific formation process of @ C is as follows,

(2) Then in the initial stage of calcination in the closed space, feSiF ₆ .6H ₂ O is firstly removed from the combined water and decomposed into SiF ₄ Gas and FeF ₂ After the temperature rises to the PAN melting point of 317 DEG CUnder the Coulomb force, PAN rapidly melts and begins to take up positively charged FeF near the embedding ₂ Crystal grains and cyclization reaction simultaneously;

3. A method for preparing coral-shaped carbon-coated ferrous fluoride as defined in claim 1, wherein: in step 1, pale green FeSiF ₆ .6H ₂ Powder O and PAN powder according to 8:1 to deionized water.

4. A method for preparing coral-shaped carbon-coated ferrous fluoride as defined in claim 1, wherein: in the step 1, the magnetic stirring time is 40min.

5. A method for preparing coral-shaped carbon-coated ferrous fluoride as defined in claim 1, wherein: light green FeSiF in step 1 ₆ .6H ₂ The preparation process of the O powder is as follows,

S101：

the reduced iron powder and the fluosilicic acid aqueous solution are mixed and stirred for 24 hours at room temperature, and ferrous fluosilicide (FeSiF) is obtained according to the following chemical formula ₆ ·6H ₂ O) precipitation of the aqueous solution with an excess of Fe;

Fe+H ₂ SiF ₆ (acid solution) →FeSiF ₆ ·6H ₂ O (Room temperature)

S102：

FeSiF prepared by S101 ₆ ·6H ₂ Centrifuging O aqueous solution, separating upper layer pale green clear liquid, evaporating at 110deg.C to dryness to obtain pale green FeSiF ₆ ·6H ₂ O powder.

6. A coral-like carbon-coated ferrous fluoride, characterized by: coral-shaped carbon-coated ferrous fluoride produced by a process for producing the coral-shaped carbon-coated ferrous fluoride of any one of claims 1-5.

7. An application of coralloid carbon-coated ferrous fluoride, which is characterized in that: the positive electrode of a lithium ion battery or a sodium ion battery, which is obtained by the method for producing coral-shaped carbon-coated ferrous fluoride according to any one of claims 1 to 5.