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In Situ Generated MWCNT-FeF3.0.33 H2O Nanocomposite Toward Stable Performance Cathode Material For Lithium Ion Batteries

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A hydrated iron(III) fluoride (FeF3. 0.33 H2O) and hydrated multi-walled carbon nanotubes- iron(III) fluoride (MWCNT-FeF3.0.33 H2O) composites were prepared by a simple two-step method. Firstly, a wet chemistry reaction between iron(III) nitrate nonahydrate (Fe(NO3)3*9H2O) and ammonium fluoride (NH4F). The thermal decomposition of the mixture in absence and presence of MWCNTs at 200 ᵒC under argon atmosphere forms FeF3. 0.33 H2O and MWCNT-FeF3.0.33 H2O. Powder X-ray diffraction, Raman spectroscopy, and scanning electron microscopy confirmed the formation of both FeF3. 0.33 H2O and MWCNT-FeF3.0.33 H2O composites, with well-distributed hexagonal shape structure with particle size ranging from 500 to 650 nm. Cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge-discharge tests in half-cell using Li metal as counter and reference electrode and 1 M LiPF6 in mixture of organic carbonates electrolyte. It was found that the battery delivers a constant voltage of 2.95 V. CV results showed that MWCNTs-FeF3.0.33 H2O composite exhibits reversible and reproducible electrochemical conversion reactions, and stabilized solid–electrolyte interface during cycling. Galvanostatic charge-discharge displayed an irreversible lithiation/delithiation processes in the first cycle due to the decomposition of the electrolyte and the formation of SEI. However, specific charge capacity was at around 200 mAh/g (greater than commercial lithium cobalt oxide cathodes 140 mAh/g) for the 20 first GCD cycles with a coulombic efficiency very close to 100 %. The excellent electrochemical performance makes from MWCNTs-FeF3.0.33 H2O a promising cathode material for LIBs in applications requiring high energy density and long cycling stability.

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Introduction

Lithium-ion batteries (LIB) industry is going through fast expansion, and are considered to be the first choice for portable electrochemical energy storage. Lithium-ion batteries have become the dominant power sources for mobile electronic devices because they came into use. These include laptops, mobile phones and some portable medical devices. Comparing LIB with other rechargeable batteries, it have the highest energy density. So developing their cost and improving their performance can greatly broaden their uses, applications and make or invent new technologies that depend on energy storage.

LIB technology is built on intercalation chemistry, intercalation in Li-ion batteries will take place only while charging or discharging, not during inactive (idle) state or when it is dead. Intercalation usually known as a reversible process. While discharging, the positive Li-ion (Li+) moves from the negative to the positive electrode, through the electrolyte solution. In contrast, the opposite process will occur during charging, that is why it is called a reversible process (Fig1. (a, b)).

Nowadays, intercalation process is much used, highly commercial. But, many researches prove that intercalation cannot be used in the future due to the law energy storage application it have. In those days, batteries performance should be improved to increase its energy storage. To solve the intercalation problems and increase the energy storage, a conversion reaction should be used. In intercalation cathodes, a new chemical bond will be created by breaking of conversion material during extraction and insertion of Li-ion. In general, Li chemistry has two types of conversion reaction: (M′ = cation, M = reduced cation material, X′ = anion)Type A (true conversion):M′Xz + yLi ↔ M + zLi(y/z)X(1)Type B (chemical transformation): yLi + X′ ↔ LiyX,(2)For the chemical transformation (type B), it transforms one single phase to another single phase. Usually type B is not used a lot in cathodes, lithiation (a reaction with lithium or an organolithium compound) of Te, Br, S, Se and I is represented in type B. In addition, reactions that takes place in type B will have very high volume capacities (theoretically). For the true conversion (type A), M′ are transition metal ions, like Fe3+, Ni2+, Cu2+, etc., while X′ are halogen ions such as F-,Cl-, Br-, etc.. While using true conversion method (type A), a metal fluoride conversion is the best solution to increase the energy storage. But, metal fluoride alone will contain few problems that should be solved before using it. Metal fluoride has very low conductivity, and that will cause a continuous changing in the volume of the material, so that will cause degradation for the material. There is four main challenges facing the conversion material method that need to be resolved before the desired performance characteristics. Low conductivity, volume change, voltage hysteresis and undesirable interaction with electrolyte (part of the metal will dissolve in the electrolyte) are the challenges that need to be solved. In comparison to sulfide and oxide cathodes, fluoride cathodes take a prominent or outstanding place for storage and energy conversion that is due to it is high electronegativity. Metal tri-fluorides (MF3) have high voltage cathode, and also large capacity cathode. Currently, phospho-olivine LiFePO4 is the most promising metal cathode for the Li-ion batteries because of it is large theoretical capacity. However, Iron (III) fluoride (FeF3) have theoretical specific capacity equals to 237mAh/g, which is beyond LiFePO4 (170 mAh/g).

In recent times, many trials to prepare nano-sized metal fluorides using supercritical-fluid technologies, pyrolysis and precipitation showed poor electrochemical performance, because of the inhomogeneity of structure and the composition. Thus, it can be concluded that the morphology of the carbon sources used, determines how able is metal fluoride cathode to keep the capacity during conversion reaction with lithium constant. Additional understanding of the reaction mechanism during charge/discharge (lithiation/delithiation) of fluoride-based cathode is needed to stop or reduce the capacity fading and voltage decay of carbon-metal fluoride nanocomposites Li-ion cathodes. So, choosing a good preparation method could improve the electrochemical performance of carbon-metal fluoride cathodes. In this work, a simple method of preparation of iron (III) fluoride-multiwalled carbon nanotubes (FeF3.0.33 H2O – MWCNT) nanocomposite cathode material was synthesized. FeF3.0.33 H2O nanoparticles were prepared by reaction of Iron (III) nitrate nonahydrate (Fe(NO3)3*9H2O) with ammonium fluoride in presence of water. After addition of multiwalled carbon nanotubes (MWCNTs) to the aqueous solution, the suspension formed is annealed under argon atmosphere at 200 C. The as-prepared MWCNT-FeF3.0.33 H2O nanocomposites were tested in half-cell Li-ion batteries with pure Li metal as anode.

Experimental section.

Chemicals

Iron(III) nitrate nonahydrate (Fe(NO3)3*9H2O) was purchased from VWR chemicals. Aqueous solution of ammonium fluoride, NH4F used as fluorinating agent which was purchased from Fluka. Organic carbonates (dimethyl carbonate, diethylene carbonate, and ethylene carbonate) were of analytical grade and obtained from Sigma Aldrich. Li ribbon (thickness x W 1.5 x 100 mm2, 99.9% trace metals basis) from sigma Aldrich was used as anode material. Lithium hexafluoro phosphate LiPF6 (battery grade, 99.99% trace metals basis) product of Aldrich was used as supporting electrolyte.

2. Synthesis of FeF3.0.33 H2O

A 3.67 g of Iron(III) nitrate nonahydrate (Fe(NO3)3*9H2O) was dissolved in 50 ml of water. 5.4 ml of ammonium fluoride (NH4F) was added to the solution drop by drop. The solution was kept under stirring and heating (at 90 C) for 2 hours until it become a gel. The mixture was then annealed in furnace tube (MTI, OTF-1200X) under argon atmosphere at 200 C for 2 hours.2.3. Synthesis of MWCNT-FeF3.0.33 H2O composite. A 3.67 g of Iron(III) nitrate nonahydrate (Fe(NO3)3*9H2O) was dissolved in 50 ml of water. 5.4 ml of ammonium fluoride (NH4F) was added to the solution drop by drop. After addition of NH4F, 0.5 g of MWCNTs was added to the solution under vigorous stirring. The solution was kept under stirring and heating (at 90 C) for 2 hours until it become a gel. The mixture was then annealed in furnace tube (MTI, OTF-1200X) under argon atmosphere at 200 C for 2 hours.

Spectroscopy and Morphology Characterizations

The composite of FeF3.0.33 H2O and MWCNT-FeF3.0.33 H2O were characterized by X-ray diffraction (PAnalytical Empyrean X-ray diffractometer) at a scan rate of 2°/min. The morphology of synthesized samples was observed by Scanning Electron Microscopy (SEM, FEI NOVA NANOSEM 450) and by Energy Dispersive X-Ray Analysis (EDX) mapping.

Coin Cells preparation

The cathode was fabricated by mixing MWCNT-FeF3.0.33 H2O composite with polyvinylidene fluoride (PVDF, 5wt.%) in Nmethyl-2-pyrrolidone (NMP) using a ball miller homogenizer for 1 h at 400 rpm. After totally mixing, the above slurries were uniformly spread onto an aluminum foil (as the current collector) using a doctor blade.

The as-prepared MWCNT-FeF3.0.33 H2O composite was directly cut into disks and assembled inside argon-filled glove box (MTI, EQ-VGB-6-LD) with gas purification and digital control systems with an oxygen and water content less than 0.1 ppm using Li metal disks as the counter and reference electrodes and a polypropylene membrane as a separator. 1M LiPF6 dissolved in 1:1:1 (v/v/v%) EC/DMC/DEC mixture was used as electrolyte. 2.6. Electrochemical Testing. Open circuit potential (OCP) test of Li/MWCNT-FeF3 battery (CorrTest electrochemical workstation) was performed. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on cells using a potentiostat/galvanostat (CorrTest electrochemical workstation).The impedance was carried out in potentiostatic mode at opencircuit potentials at a fully charged state. The amplitude of the AC signal was set at 5mV and the frequency ranged from1MHz to 10mHz. Galvanostatic charge-discharge tests of Li//MWCNT-FeF3 batteries were conducted using MTI 8 channel battery analyzer (BST8-WA, 0.005–1mA, up to 5 V) with adjustable cell holders in the voltage range 1.5–4.5 V at various rates.

Results and Discussion

MWCNT-FeF3.0.33 H2O composite were prepared by a simple wet chemistry route based on ion exchange, and the fluorinating agent used in the preparation was ammonium fluoride.

The reaction of Fe(NO3)3*9H2O with NH4F forms a yellow clear solution of FeF3.0.33 H2O (Eq. (3)). The addition of MWCNTs to MnSiF6 acidic solution produces a homogeneous suspension after vigorous stirring overnight.(3)Powder X-ray diffraction (XRD) patterns of FeF3.0.33 H2O and MWCNT-FeF3.0.33 H2O composite are shown in Fig 2. The XRD spectra coincide with XRD spectrum of FeF3. 0.33 H2O confirming the formation of hydrated FeF3 during the reaction between Fe(NO3)3 and NH4F in aqueous solution as given by the following equation: The average grain size of FeF3.0.33 H2O particles can be calculated from the most intense line of XRD spectrum using Scherrer formula: With the particle shape factor K=0.9, the X-ray wavelength λ=1.5406Å, θ is the diffraction angle for the most intense peak and β is the experimental full width at half-maximum (FWHM) of the same peak. The calculated grain size was about 610-665 nm, which proves that the method used here led to the formation nanostructured hexagonal FeF3-MWCNTs composites.

The morphology of FeF3.0.33 H2O and MWCNT-FeF3.0.33 H2O composite are given in Fig 3. SEM images (in two different magnifications) show that FeF3.0.33 H2O and MWCNT-FeF3.0.33 H2O composite are in a well-distributed hexagonal shape structure with particle size ranging from 500 to 650 nm, which is in good agreement with particle size calculated from XRD data.

SEM-EDX mapping and EDX spectrum results of FeF3.0.33 H2O show the distribution of oxygen, iron and fluoride with weight percent of 4.07 %, 46.77 %, and 49.16 %, respectively (Fig 4.). These weight percentages fit well with the formula unit FeF3.0.33 H2O (Table 1), considering that H element cannot be detected by SEM-EDX. However, EDX spectrum of MWCNT-FeF3.0.33 H2O shows the presence of carbon in addition to O, Fe, and F in SEM mapping. The atomic percent of the elements are 24.57 % C, 4.21 % O, 41.96% F, and 29.27 % Fe. These results support the XRD data of part 1.3 and confirm the formation of MWCNTs-FeF3.0.33 H2O composite.

The electrochemical performance of MWCNT-FeF3.0.33 H2O composite as cathode material in half coin cells using Li metal as a reference and counter electrode and 1M LiPF6 in EC/DMC/DEC (1/1/1 v/v/v) as electrolyte, was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Fig 5a presents the CVs of MWCNT-FeF3.0.33 H2O electrode between 0.5 and 4.0 V versus Li+/Li at scan rate of 1 mVs-1.In the first oxidation scan, an anodic peak at 3.3 V vs Li+/Li that can attributed to the delithiation of Fe and formation of FeF3. In the return scan, a reduction peak appeared at around 1.8 V vs Li+/Li corresponding to the lithiation of FeF3 leading to the formation of Fe and LiF. In the first cycle, a broad anodic peak at 3.3 V vs Li+/Li appeared indicating the delithiation of Fe and formation of FeF3. In the return cycle, a reduction peak appeared at 1.8 V vs Li+/Li corresponding to the lithiation of FeF3 leading to the formation of Fe and LiF; these peaks can be paired with the anodic peaks observed at 3.3 V vs Li+/Li. In the second cycle, the anodic peak was exactly at the same potential of the first cycle, 3.3 V vs Li+/Li. In the second return cycle, the cathodic peak was also identical to the first cycle, and it appeared at 1.8 V vs Li+/Li. For subsequent CVs (CV 5 and CV 10), the anodic peak appeared also at 3.3 V vs Li+/Li. Simultaneously, the cathodic peak at 1.8 V vs Li+/Li decreased in intensity and shifted to higher potentials. These results indicated that the electrochemical conversion reactions occurs in one stage; Fe is oxidized directly to FeF3.

The conversion of FeF3 to Fe with formation of LiF happened in one step (the transfer of 3 e simultaneously met in literature). It appears that the formation of solid-electrolyte interface (SEI) during cycling limited the formation of iron(III) fluoride; whereas an enhanced reversibility was observed for the electrochemical conversion of Fe to FeF3 and of FeF3 to Fe (). Electrochemical impedance spectroscopy (EIS) test was also performed before and after CV tests to detect interfacial changes and especially to investigate the kinetics of growth of SEI films. Nyquist plots of Li//MWCNT-FeF3battery using 1M LiPF6 in EC/DMC/DEC (1/1/1 v/v/v) as electrolyte (Fig 5b) plots presents impedance arcs starting from same value of about 10 Ohms; such value is considered reasonable for carbon composite materials. This indicates that the incorporation of FeF3 inside the carbon matrix significantly enhanced the electrical conductivity compared to poorly conductive pure FeF3. The presence of one semicircle in all the Nyquist plots indicates that the equivalent electric circuit comprises a resistance in parallel with capacitance. A significant increase of diameter of the semicircle was observed before and after the first CV. A semi-circle diameter of 250 Ohms was detected before the first CV, while a diameter of 810 Ohms was observed after the first CV, this significant increase of diameter indicate the formation of a thin SEI film. After the second CV, the diameter continue to increase to reach 975 Ohms, but it started to decrease after the fifth and the tenth CV to 745 Ohms and 230 Ohms, due to the widening of the channels and pathways for Li+ ions and electrons transfer that develop inside the composite cathode and through SEI during cycling.

The changes of open circuit potential (OCP) with time of freshly prepared and uncycled Li//MWCNT-FeF3 battery was taken for a period of one hour. Fig 6a shows that the OCP value remained constant during the measurement and equal to 2.945 V vs Li+/Li, which is very close to the values reported in literature for FeF3 and its carbon composites (3.0 -3.5 V vs Li+/Li).[13] The electrochemical behavior of MWCNT-FeF3 composite cathode was also studied via galvanostatic charge-discharge (GCD) experiments. The galvanostatic charge-discharge tests were performed in the voltage range 1.5 to 4.5 V versus Li+/Li. Fig 6b shows charge-discharge curves and charge discharge specific capacity changes during cycling of MWCNT-FeF3 in half-cell using Li as a reference and a counter electrode 1M LiPF6 in EC/DMC/DEC (1/1/1 v/v/v) as electrolyte. The first discharge cycle presents a first reduction plateau at 2.75 V vs Li+/Li. These results confirmed the existence of one stage during the discharge of MWCNT-FeF3 cathode (lithiation process), which starts with the electrochemical reduction of FeF3 then reduced to Fe with formation of LiF. A good accordance between the CVs results and the first GCD plot was perceived. In the later discharge cycles, the plateau appeared at 2.5 V vs Li+/Li.The first charge cycle presents no plateau and only a shoulder in the increase of voltage between 1.5 and 3.5 V vs Li+/Li corresponding to the delithiation process (Fe is oxidized to FeF3). Similar results were observed in the subsequent charge cyclesThe specific charge and discharge capacities of 210 mAh/g and 375 mAh/g were measured with CE =178% (Fig 6c). The irreversible capacity and the higher CE than 100 % indicates the decomposition of the electrolyte and formation of SEI during first discharge cycle. After the second cycle, the charge specific capacity was retained at 200 mAh/g with CE very close to 100% during the 20 first cycles indicating risibility of the electrochemical conversion reactions.

Conclusion

In this work, FeF3.0.33 H2O and MWCNT-FeF3.0.33 H2O composites were synthesized using simple, two-step method including a wet chemistry reaction followed by a thermal decomposition. The reaction of Fe(NO3)3*9H2O and NH4F, without and with the presence of MWCNTs forms a FeF3.0.33 H2O and MWCNT-FeF3.0.33 H2O composites after the thermal treatment under argon atmosphere at 200C. X-ray diffraction and scanning electron microscopy confirm the formation of the partially hydrated FeF3 (FeF3.0.33 H2O). In addition, scanning electron microscopy (SEM) micrographs confirmed the formation of nanostructured compounds. A detailed SEM mapping and EDX analysis confirmed the formation of the partially hydrated FeF3 (FeF3.0.33 H2O). Both prepared compounds were selected to be tested as cathode material in Lithium ion batteries (LIBs). Specific charge and discharge capacities of 210 and 375 mAh/g were measured in the first GCD cycle, respectively. The specific charge capacity was retained at around 200 mAh/g (greater than commercial lithium cobalt oxide cathodes 140 mAh/g) for the 20 first GCD cycles with a coulombic efficiency (CE) very close to 100 % indicating reversible electrochemical conversion reactions (). In addition, GCD tests showed that MWCNTs-FeF3.0.33 H2O composite displayed an irreversible lithiation/delithiation processes in the first cycle due to the decomposition of the electrolyte and the formation of SEI. Considering the simple method of preparation and the excellent electrochemical performance and cycling stability of MWCNTs-FeF3.0.33 H2O composite, this composite material could be a promising cathode materials for LIBs in applications requiring higher energy densities and long cycling stability.

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