In situ generated MWCNT-FeF3·0.33 H2O nanocomposites 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 the absence and presence of MWCNTs at 200 °C 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 (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge discharge (GCD) were performed to test MWCNT-FeF3·0.33 H2O composite cathode in half-cell using Li metal as counter and reference electrode and 1 M LiPF6 in mixture of organic carbonate 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. GCD profile 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 498 mAh/g (greater than commercial lithium cobalt oxide cathodes ~ 140 mAh/g) after 50 cycles with an average Coulombic efficiency of 95%. The excellent electrochemical performance makes from MWCNTs-FeF3·0.33 H2O a good candidate cathode material to replace conventional materials for LIBs in applications requiring high energy density and long cycling stability.
KeywordsLi ion batteries Iron(III) fluoride Multi walled carbon nanotubes Electrochemical performance Columbic efficiency
Lithium ion batteries (LIBs) are gaining more and more interest and they are being considered as the first-choice electrochemical energy storage in the field of mobile electronic devices including laptops, mobile phones, and portable medical devices [1, 2]. This is due to their higher energy density, longer cycling lifetime, lower self-discharge, and resistance compared to other rechargeable batteries. However, to meet the requirements of integrating renewable energy sources and replacing internal combustion car engines with electrical/hybrid systems, significant improvement in the energy density as well as reduction in the costs beyond the state-of-the art LIBs are required [3, 4].
The current generation of Li-ion batteries is based on intercalation electrodes, where the anodes have been comprised of graphitic carbon, while the cathodes have been produced with lithium metal oxides, such as lithium cobalt oxide (LCO), lithium nickel manganese cobalt oxide (NMC), lithium nickel aluminum cobalt oxide (NCA), to name a few. A reversible intercalation/deintercalation mechanism takes place during charge and discharge . During charge, Li+ ions move from the anode (negative electrode) to intercalate between the layered lithium metal oxide (positive electrode) through the electrolyte solution, while the opposite process occurs during charge. At full charge, graphite can intercalate one Li atom per six carbon atoms (LiC6) for a theoretical capacity of 372 mAh/g; however, a full discharge, LCO for example, can intercalate one Li atom per one formula unit for a theoretical capacity of 272 mAh/g . The intercalation/deintercalation of a maximum of one atom per unit makes LIBs applications limited to small electronic devices. Therefore, highly needed further improvements in the performance characteristics of Li-ion batteries are largely dependent on our ability to develop novel materials with greatly improved Li ion storage capacities.
Conversion and alloying electrodes can be a good alternative to conventional electrodes because they can exchange greater number of Li ions per unit during charge/discharge. Fluoride-based cathodes offer an outstanding technological potential due to high capacities and low cost [6, 7, 8, 9, 10]. Among the low-cost (not Ag-based), high voltage, environmentally friendly (Co-free) fluoride cathode, some of the highest theoretical energy densities are exhibited by FeF3 with a specific capacity of 712 mAh/g . The low electrical conductivity of FeF3 initially resulted in the low utilization of their theoretical capacity. However, reduction in the grain size of FeF3 and the incorporation of the nano-sized grains into a conductive matrix of carbon or even selected metal oxides allowed significant enhancements of the cathode utilization (some close to the theoretical capacity) [10, 11, 12, 13]. FeF3 and C-FeF3 composite cathode materials have been produced via mechanochemical (high-energy ball milling) synthesis routes [10, 11, 12, 13]. While this is a versatile, scalable, and promising process, it does not allow sufficient uniformity and control over the microstructure of the produced materials at the nanoscale. The architecture of the composites as well as the size and shape of their building blocks, the microstructure (structural ordering) of the individual components, and their porosity are known to have a profound effect on the electrochemical stability . Recently, hydrated FeF3 (FeF3·3H2O, FeF3·H2O, FeF3·0.33 H2O) prepared by solvothermal and wet chemistry methods showed excellent electrochemical performance as cathode material for LIBs [14, 15, 16, 17, 18, 19].
In this work, a simple method of preparation of multiwalled carbon nanotube-iron (III) fluoride-(MWCNT-FeF3·0.33 H2O) nanocomposite cathode materials were introduced. The addition of ammonium fluoride (NH4F) to an aqueous suspension of MWCNT and iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O) forms a precipitate of hydrated FeF3 nanoparticles that decorate MWCNT and incorporate inside MWCNT network. After filtration, washing, and drying in vacuum oven at 80 °C, the solid mixture was annealed under argon atmosphere at 200 °C. The same procedures were used to prepare FeF3·0.33 H2O without addition of MWCNT. The as-prepared FeF3·0.33 H2O and MWCNT-FeF3·0.33 H2O nanocomposites were tested in half-cells using Li metal as counter and reference electrode.
2 Experimental section
Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O) was purchased from VWR chemicals. Ammonium fluoride (NH4F) was obtained from Fluka. Organic carbonates (diethylene carbonate DEC, ethylene carbonate EC, and dimethyl carbonate, DMC) were purchased from Sigma Aldrich. Li ribbon (99.9% trace metals basis) and lithium hexafluorophosphate LiPF6 (the battery grade, ≥ 99.99% trace metals basis) were also obtained from Sigma Aldrich. MWCNT was purchased from Adnano technologies (Purity 99%, Diameter 10–15 nm, Surface area ~ 400 m2/g, length ~ 5 μm).
Synthesis of FeF3·0.33 H2O
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) (5 M) was added drop by drop to iron(III) aqueous solution. The mixture was kept under stirring at 90 °C for 2 h until the solvent was almost evaporated. After drying in vacuum oven at 80 °C overnight, the solid mixture was annealed in tube furnace (MTI, OTF-1200X) under the argon atmosphere at 200 °C for 2 h.
Synthesis of MWCNT-FeF3·0.33 H2O composite
3.67 g of Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O) was dissolved in 50 ml of water, then 0.5 g of MWCNT was added under stirring to form a homogeneous suspension. 5.4 ml of ammonium fluoride (NH4F) was then added to the solution as drop by drop. After the addition of NH4F, the mixture was kept under stirring at 90 °C for 2 h until a gel-like suspension was formed. The gel-like suspension was dried overnight at 80 °C in vacuum oven. The dried solids were annealed at 200 °C under argon flow in tube furnace for 2 h.
Spectroscopy and morphology characterizations
The determination of crystalline structure and solid phases were performed by X-ray diffraction (PAnalytical Empyrean X-ray diffractometer) at a scan rate of 2°/min between 10 and 90°. The morphology and elemental analysis of the as-prepared samples were analyzed by scanning electron microscopy (SEM, FEI NOVA NANOSEM 450) and energy dispersive X-Ray (EDX) spectroscopy.
Coin cell preparation
The active materials were mixed with the binder polyvinylidene fluoride (PVDF, 5% in weight). When no carbon was contained in the active material, carbon super P (25% in weight) was added as conductive additive. After addition of 1–2 ml N-methyl-2-pyrrolidone (NMP), the mixture was ball-milled for 1 h at 400 rpm to make homogeneous slurries. These slurries were homogeneously spread onto aluminum foil (used as a current collector) using doctor blade casting machine. The as-prepared electrodes were dried under vacuum at 80 °C for 6 h and then punched into disks. CR-2032 coin cells were assembled inside argon-filled glove box (MTI, EQ-VGB-6-LD) with gas purification and a digital controller system (oxygen and water content less than < 0.1 ppm) using lithium metal disks as counter and reference electrodes and a polypropylene membrane (Celgard, 2400) as the separator. 1 M LiPF6 dissolved in (1:1:1 (v/v/v) EC/DMC/DEC mixture was used as an electrolyte.
Open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) tests were performed using galvanostat–potentiostat (CorrTest electrochemical workstation). EIS measurements were carried out at open circuit potentials in fully charged states. The amplitude of AC signals were set at (5 mV) with a frequency range from 0.01 to 105 Hz. Galvanostatic charge-discharge (GCD) tests were performed using MTI 8 channel battery analyzer (BST8-WA, 0.005–1 mA, up to 5 V) with adjustable cell holders in a voltage range 1.4–4.0 V at different cycling rates.
3 Results and discussion
The addition of MWCNTs to the aqueous solution of Fe(NO3)3·9H2O followed by the addition of NH4F co-precipitate FeF3 on the surface of MWCNTs. The precipitate formed in the absence and in the presence of MWCNTs was washed several times with deionized water to remove NH4NO3, and then separated by centrifugation and dried at 80 °C in vacuum oven. The collected solids were then heat-treated in tube furnace under Argon flow at 200 °C.
Where K is a factor that represents the particle shape with a value equal to 0.9 and the X-ray wavelength is λ = 1.5406 Å. In addition, θ can be defined as diffraction angle for almost intense peak. β is the experimental full width at half-maximum of the same peak. The calculated grain size was 610 nm for FeF3·0.33 H2O and MWCNT-FeF3·0.33 H2O for 665 nm, which confirms the formation of nanostructured particles.
Weight percent for all the elements of the formula unit FeF3·0.33 H2O
MW: 118.8 g/mol
Electrochemical impedance spectroscopy (EIS) test was also performed before and after CV tests to detect interfacial changes and specially to investigate the growth of SEI films. Nyquist plots of Li//MWCNT-FeF3 battery using 1 M LiPF6 in EC/DMC/DEC (1/1/1 v/v/v) as electrolyte present impedance arcs starting from same value of about 10 Ohms (Fig. 4b); 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 after the first CV. A semi-circle diameter of 250 Ohms was measured before the first CV, while the diameter increased to 810 Ohms after the first CV; this significant increase of the impedance arc diameter indicates the formation of a thin SEI film. After the second CV, the diameter continues to increase reaching 975 Ohms, but it started to decrease after the fifth and the tenth CV to 745 and 230 Ohms, indicating the enhancement of charge transfer kinetics by creating new channels inside the SEI film and shortening the pathways for Li+ ions and electrons during cycling. The changes of open-circuit potential (OCP) with time of freshly prepared and uncycled Li//MWCNT-F FeF3·0.33 H2O battery was taken for a period of 1 h. The OCP remained constant 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) .
Specific charge and discharge capacities of 597 mAh/g and 902 mAh/g were measured with CE = 151% (Fig. 5c) for MWCNT-FeF3·0.33 H2O. The irreversible capacity and the greater CE than 100% indicate the decomposition of the electrolyte and formation of SEI during the first discharge cycle [16, 17, 18, 19, 20, 21, 22]. After the second cycle, the charge specific capacity decreased to reach 497, 488, 470, and 446 mAh/g after 50, 100, 120, and 150 cycles, respectively. After 165 cycles, the specific charge capacity retention was 74%. This can be due to partial dissolution of the active materials (mainly Fe and LiF) in the electrolyte as mentioned in several reports [20, 21, 22, 23]. CE was greater than 95% during the 165 cycles demonstrating good reversibility for the electrochemical conversion reactions. The consecutive rate capability tests (in Fig. 5d) also show good cycling stability at each rate. The capacity was recovered, even after high capacity rate, indicating that high capacity rates did not damage the electrode. As the current density increases, the first specific charge capacity decreases from 718 mAh/g at 70 mA/g to 597 mAh/g, 418 mAh/g, 207 mAh/g, and 698 mAh/g at 140 mA/g, 280 mA/g, 540 mA/g, and 70 mA/g, respectively. Importantly, MWCNT-FeF3·0.33 H2O composite cathodes showed a high Coulombic efficiency (CE) for different capacity rates with an average CE after 20 cycles exceeding 99%, which demonstrates a good electrochemical reversibility. It is noticeably marked that MWCNT-FeF3·0.33 H2O composite cathode showed better electrochemical characteristics in terms of specific capacity, cycling stability, and rate performance than FeF3·0.33 H2O/C nanocomposites prepared by ball milling and solvothermal methods reported by Wei et al. and Zhang et al. [15, 16].
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 FeF3·0.33 H2O and MWCNT-FeF3·0.33 H2O composites after the thermal treatment under argon atmosphere at 200 °C. X-ray diffraction and scanning electron microscopy confirm the formation of the partially hydrated FeF3 (FeF3·0.33 H2O). A detailed SEM mapping and EDX analysis confirmed the formation of the partially hydrated FeF3 (FeF3·0.33 H2O). GCD tests showed that MWCNTs-FeF3·0.33 H2O composite displayed irreversible lithiation/delithiation processes in the first cycle due to the decomposition of the electrolyte and the formation of SEI. The specific charge capacity decreased from 597 to 498 mAh/g after 50 cycles (greater than commercial lithium cobalt oxide cathodes ~ 140 mAh/g) with an average Coulombic efficiency (CE) of 95% indicating good reversibility of the electrochemical conversion reactions (FeF3 + 3Li+ + 3 e ⇆ Fe + 3LiF). 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 material for LIBs in applications requiring high energy densities and long cycling stability.
The contents of the study are solely the responsibility of the authors and do not necessarily represent the official views of the Qatar National Research Fund.
This work was funded by a grant from the Qatar National Research Fund under its National Priorities Research Program award number NPRP7-567-2-216.
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