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MWCNTs nanohybrid as anode material for lithium-ion battery
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NiFe2O4 nanoparticles/MWCNTs nanohybrid as anode material for lithiumion battery Muhammad Mujahida,b, Rafi Ullah Khana, Muhammad Mumtazb,∗, Mubasherb, Sumair Ahmed Soomroc, Shafiq Ullahd a
Department of Mechanical Engineering, Faculty of Engineering and Technology (FET), International Islamic University (IIU), Islamabad, 44000, Pakistan Materials Research Laboratory, Department of Physics, Faculty of Basic and Applied Sciences (FBAS), International Islamic University (IIU), Islamabad, 44000, Pakistan c National University of Sciences and Technology (NUST), Islamabad, Pakistan d National Engineering and Scientific Commission, Islamabad, Pakistan b
A R T I C LE I N FO
A B S T R A C T
Keywords: NiFe2O4/MWCNTs nanohybrid Co-precipitation Toluene Lithium-ion battery Dielectric properties
Nickel ferrite (NiFe2O4) nanoparticles were first prepared by co-precipitation route and then they were combined with multiwall carbon nanotubes (MWCNTs) using ultra-sonication assisted method. A facile two-step method was used to synthesize NiFe2O4/MWCNTs nanohybrid as an anode material to modify the electrochemical performance of lithium-ion battery. For the first-time, Toluene was used as a dispersive/linkage medium for the synthesis of this nanohybrid material. The formation of inverse spinel cubic structure with no impurity was investigated by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The average particle size of spherical nanoparticles was found about 36 nm NiFe2O4nanoparticles were embedded on the surface of MWCNTs. This nanohybrid material showed high specific capacity (i.e. 1302 mAh/g) and better cyclic performance (i.e. 871 mAh/g) after 25 cycles at 0.2 C-rate. More importantly, this showed excellent rate capability (i.e. 631 mAh/g) even at high 5 C-rate and returned to the initial capacity at 0.2C by giving 97% columbic efficiency.
1. Introduction Due to the increasing consumption and fast depletion of fossils fuels, the focus of the scientists and the researchers has been shifted towards capitalization of renewable energies i.e. wind energy, tidal energy, solar energy and others. To achieve widespread applications of these renewable energies, high energy storage devices and conversion technology is required. Subsequently, lithium-ion batteries attracted more attention because of its design flexibility and high specific energy storage [1,2]. Material of anode plays the key role that influences the net performance of lithium-ion batteries. It is very necessary to manufacture the anode of lithium-ion batteries from robust, non-toxic and economical materials to overcome the rising demand of high energy storage devices. For this purpose, nano-structured transition metal oxides (i.e. Mn3O4, Co3O4, Fe2O3, SnO and SiO2 etc) have been widely used as a promising anode materials [3–7]. Moreover, divalent metalbased ferrites (i.e. MFe2O4 where M = Co2+, Ni2+, Cu2+, Mn2+etc) got more attention because of their low cost, environment friendly and most importantly their high lithium-ion storage capacity (i.e. more than 900 mAh/g), which is much higher than commercially used graphite
∗
anode (i.e. 372 mAh/g). Among these ferrites, NiFe2O4 material is considered as a soft magnetic material having cubic spinel structure with prominent electrochemical properties due to its massive surface atoms percentage, large specific surface area and insulating behavior, which often make it suitable as a charge storage material. The only drawback of these oxides nanoparticles are their low conductivity and agglomeration that caused severe capacity fading during charging/ discharging cycles [8,9]. This problem can be solved by combining these ferrites materials with some other materials having excellent electrical conductivity. Carbonaceous nano-materials such as carbon nanotubes (CNTs) and graphene nano-sheets, have already been proven of having good chemical inertness, superior electrical conductivity and high contact area for utilization in energy storage devices [8]. CNTs have already been widely used by researchers due to its low cost and easy availability. Keeping in mind the properties of both NiFe2O4 and CNTs, it's rather natural to combine both into a nanohybrid with the complimentary properties of both components. CNTs usually provide structure stability when used as a filler material with nano-structured transition metal oxides [10]. Most importantly, the insertion of CNTs prevent the
Corresponding author. E-mail address: [email protected] (M. Mumtaz).
https://doi.org/10.1016/j.ceramint.2019.01.160 Received 21 December 2018; Received in revised form 15 January 2019; Accepted 21 January 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mujahid, M., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.01.160
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2.2. Samples characterization
aggregation, expansion and contraction in volume of the nanoparticles of the electrodes during discharging and charging process [8,11]. It also works as a highly conductive matrix and creates a contact between electrode and current collector. Meanwhile, the decoration of transition metal oxides nanoparticles on the surface of the CNTs reduces the restacking of nanotubes sustaining the high surface area [12]. Due to synergistic effects by combining the high capacity transition-metal oxides nanoparticles with high surface area/conductivity of CNTs, the improvement in cyclic stability, rate capability and storage capacity in CNTs based nanohybrids for lithium-ion batteries has been observed [11,13–18]. Researchers opted various routes to synthesize the nanoparticles/ CNTs nanohybrids [15,19–23]. The previous studies primarily focused on embedding nanoparticles on CNTs by using xylene, pyrene, benzyl alcohol and phenylphosphoric acid as derivative linkages [23–25]. Toluene has been used for the first time as dispersive medium in the current research work. Also, the use of MWCNTs with ferrite nanoparticles to synthesize nanohybrids has not yet been studied for lithium ion batteries. In literature, single wall carbon nanotubes (SWCNTs) have already been used for such types of nanohybrids. But the drawbacks of SWCNTs are that they are very expensive and their nanohybrids have shown less rate performance. The semiconducting nature of SWCNTs leads to slow kinetics, which can be responsible for poor rate capability, while MWCNTs are considered to be metallic in nature and thus are expected to offer better rate capability, as compared to SWCNTs [10]. That's why, the main focus of this study is to improve the electrochemical performance, most preferably rate capability as well as storage capacity. We preferred to use Toluene as dispersive medium because it has aromatic ring structure as that of xylene, with single methyl-group, π-π interaction mechanism and less density than xylene [24,25]. In this work, NiFe2O4 nanoparticles were first synthesized by co-precipitation method and then embedded on the surface of MWCNTs by sonication assisted method using Toluene as a dispersive/linkage medium to modify the storage capacity, cyclic stability and rate capability.
The crystal structure and phase formation of NiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid in powder form were investigated by Bruker D8 Advance X-rays Diffraction (XRD) equipment using CuKα (λ = 1.5405 Å) radiation. Fourier transform infrared (FTIR) spectroscopy of NiFe2O4 nanoparticles, MWCNTs and NiFe2O4/MWCNTs nanohybrid were carried out in the range from 400 to 4000 by using KBr pellets. The morphology of these nanoparticles and nanohybrid were examined by scanning electron microscope (SEM) (Model JEOL-JSM6390 and Vega3 Tescan), field emission scanning electron microscope (FE-SEM) (Model FEI Panalytical Quanta 450) and transmission electron microscope (TEM). The band-gap energy of the samples was measured by UV–visible spectroscopy (Perkin-Elmer equipment). The ac-conduction properties were performed on the pellets of these samples sintered at 450 °C for 2 h by using impedance analyzer (LCR 6500B WK model). The thermo-gravimetric analysis (TGA) was carried out at ambient temperature to 1000 °C by using NETZSCH Jupitor, model STA 449-F3 equipment. The chemical composition of the samples were determined by energy dispersive spectroscopy (EDS). 2.3. Battery testing The electrodes for Li-ions battery were fabricated of thick slurry of active material (NiFe2O4 nanoparticles, NiFe2O4/MWCNTs nanohybrid), binder polyvinylidene fluoride (PVDF) and conducting carbon in the weight ratio 8:1:1.1-methyl 2-pyrolidone (NMP) solvent was used to mix slurry completely. The thick and homogeneous slurry was coated uniformly on one side of a copper foil and heated at 100 °C for 12 h. The dried thick films of electrodes were pressed at about 1500 kPa pressure and cut into circular discs of 16 mm diameter for good adherence of electrode material on the copper foil. Pure lithium metal foil was used as counter electrode, polyethylene was used as a separator and 1 M LiPF6 solution dissolved in ethylene carbonate dimethyl carbonate (1:1) was used as electrolyte medium. These electrodes were assembled in argon containing glove box in the form of CR-2023 type coin-cell.
2. Experimental details 3. Results and discussion 2.1. Preparation of NiFe2O4/MWCNTS nanohybrid 3.1. XRD and TGA analysis Typically, the salts of metals in the form of nitrate (Sigma-Aldrich) and sodium hydroxide were used as precursors for the preparation of nano-crystalline NiFe2O4 particles by co-precipitation route. Initially, 0.1 M solution of nickel nitrate hexahydrate (i.e. Ni(NO3)2.6H2O) and 0.2 M solution of iron (III) nitrate nonahydrate (i.e. Fe(NO3)2.9H2O) were mixed with each other by stirring. The mixture was then heated at 85–90 °C and a 3 M solution of sodium hydroxide previously heated at 80 °C was added and constantly stirred for an hour at 90 °C. The product was cooled down to room temperature and washed with double distilled water several times until the pH 7 was achieved. The resultant solution was dried by overnight heating at 100 °C followed by annealing at 800 °C for 8 h to get the distinct impurity free spinel phase structure of NiFe2O4 nanoparticles. The powder was ground in mortar and pestle to eliminate agglomeration and to obtain fine nanostructures. These NiFe2O4 nanoparticles were added into 50 ml of Toluene separately and sonicated for an hour at room temperature for uniform dispersion. After the uniform dispersion of these NiFe2O4 nanoparticles in Toluene medium, 5% of MWCNTs were added in the medium. The mixture was again sonicated for 5 h using water bath sonicator. Toluene fluid was evaporated by keeping it at 100 °C leaving behind uniformly dispersed NiFe2O4/MWCNTs nanohybrid. This nanohybrid material was further homogenized using mechanical mixing (solid-state method) to avoid agglomeration. Finally the prepared NiFe2O4/MWCNTs nanohybrid was calcined at 350 °C for 2 h for the good adhesion of NiFe2O4 nanoparticles with MWCNTs walls.
The powder XRD technique was used to investigate the crystal structure and phase formation of the samples. The XRD profile of pure NiFe2O4 nanoparticles, MWCNTs and NiFe2O4/MWCNTs nanohybrid are shown in Fig. 1. The peaks (111), (220), (311), (400), (422), (511),
Fig. 1. XRD spectra of (a) MWCNTs, (b) NiFe2O4 nanoparticles and (c) NiFe2O4/MWCNTs nanohybrid. 2
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(440) and (533) confirmed the expected closed packed FCC cubic structure with Fd3m space group of NiFe2O4 nanoparticles in accordance with card number JCPDS 22–1086. The distinct appearance of peak (311) at 35.760° showed good crystallinity of the product while the FWHM showed the formation of small crystallite size, which in turn confirmed the formation of small size grains. No additional peak appeared other than that of cubic arrangement in the pattern, which showed that no impurity was present in the product material. The crystallite size calculated by Debye-Scherrer's formula from the XRD data of NiFe2O4 nanoparticles was 29 nm. In XRD spectrum of MWCNTs, the characteristics (002) and (100) peaks represented the graphitic reflection. The XRD spectrum of NiFe2O4/MWCNTs nanohybrid gave almost the same spectrum as that of pure NiFe2O4 nanoparticles and graphitic peaks was not found, which accounted for the complete anchoring of NiFe2O4 nanoparticles on the surface of MWCNTs, indicating the uniform dispersion and the effectiveness of the opted method. However, the broadening of the peaks on coating showed the decrease in crystallite size (i.e. 24 nm) and ascribed to adsorption of nanoparticles on the surface of MWCNTs, which diminished agglomeration and thus crystallite size. The bulk density ‘Db’ (i.e. the ratio between mass and bulk volume), the X-ray density ‘Dx’ (i.e. measured from XRD data) and the porosity fraction (i.e. 1 – (Db/Dx)) measured for pure NiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid are given in Table 1. Since MWCNTs were porous in nature with heptagon-pentagon pair and interstitial defects, the decrease in density and increase in porosity were expected for prepared nanohybrid. Thermo-gravimetric analysis (TGA) were performed for both the samples from room temperature to 1000 °C at a rate of 5 °C/min to quantify the volume of MWCNTs and NiFe2O4 nanoparticles in nanohybrid as shown in Fig. 2. The percentage loss of a pure NiFe2O4 nanoparticles was just 2%, while the nanohybrid showed rapid mass loss upto 9% at the range of 700 °C–900 °C, which is attributed to the oxidation of MWCNTs. Hence the amounts of the NiFe2O4 nanoparticles in NiFe2O4/MWCNTs nanohybrid are about 91% as calculated from the residual weight loss.
Fig. 2. TGA pattern of prepared samples.
Fig. 3. FTIR spectra of (a) MWCNTs (b) NiFe2O4 nanoparticles and (c) NiFe2O4/ MWCNTs nanohybrid.
3.2. FTIR spectroscopy The FTIR spectra of pure NiFe2O4 nanoparticles, MWCNTs and NiFe2O4/MWCNTs nanohybrid obtained at room temperature is shown in Fig. 3. In case of pure NiFe2O4 nanoparticles, two strong absorption bands v1 around 597 cm−1 and v2 around 408 cm−1 represents tetrahedral (A) and octahedral (B) sites, respectively [23]. These ν1 and ν2 absorption bands are in fact credited to the metal-oxygen stretching vibration mode at A and B sites. The formation of these two sub-lattices bands in the FTIR spectra confirmed the formation of single-phase spinel structure and compatible with the previously reported results [26]. In the FTIR spectra of MWCNTs, the stretching mode at 1628 cm−1 is assigned to the vibration of C = C graphitic layers, which was formed due to MWCNTs sidewall framework [27]. The broad peak center at 3439 cm−1 corresponds to the eOH (hydroxyl) stretching band [28]. In case of NiFe2O4/MWCNTs nanohybrid, a shift of bands is observed, which indicates the subsistence of inter-molecular forces between MWCNTs and NiFe2O4 nanoparticles.
3.3. Morphology and chemical analysis The structural and chemical analysis of the materials was performed by using SEM, FE-SEM, TEM and EDS. The SEM, FE-SEM and TEM images of MWCNTs, pure NiFe2O4 nanoparticles, and NiFe2O4/ MWCNTs nanohybrid at different magnification are shown in Fig. 4(a–d), Fig. 5(a and b) and Fig. 6, respectively. It can be clearly observed that pure NiFe2O4 nanoparticles are spherical in shape with average diameter of 36 nm that is in good agreement with the calculated size by Debye-Scherrer's formula. A small agglomeration can also be observed, which can be accounted for the annealing of the synthesized product owing to the diffusion of the atoms and high surface energy. However, when particles normally come into contact with each other and under favorable energy conditions, some of them grow while others decrease in size and eventually disappear. So, as a result a small number of large particles are evident from these images. The crystallite clusters cemented together can be easily visible from the FE-SEM images as shown in Fig. 5(a and b). The diameter of free MWCNTs is measured as 25–35 nm. The SEMs of NiFe2O4/MWCNTs nanohybrid without grinding and sonication and after grinding and sonication are shown in Fig. 4(c) and (d), respectively. The large numbers of nanoparticles are completely embedded on the wall surfaces of MWCNTs even after grinding as shown in Figs. 4(d), 5(b) and 6, due to π-π interaction [24,25], which confirm that our opted dispersion procedure for preparation of the nanohybrid was of high efficiency. The EDS analysis patterns of NiFe2O4nanoparticles and NiFe2O4/
Table 1 Variation of X-ray density, bulk density, volume of cell, crystallite size and porosity of NiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid. Parameters 3
X-ray Density (g/cm ) Bulk Density (g/cm3) Volume of cell (nm3) Crystallite Size (nm) Porosity (%)
NiFe2O4 nanoparticles
NiFe2O4/MWCNTs nanohybrid
5.30 3.33 579.51 29 37.2
5.16 2.49 581.62 24.5 43.6
3
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Fig. 4. SEMs of (a) NiFe2O4 nanoparticles (b) MWCNTs (c) NiFe2O4/MWCNTs nanohybrid without grinding and sonication (d) NiFe2O4/MWCNTs nanohybrid after grinding and sonication in distilled water.
where α, Eg, h, υ and b represent absorption coefficient, energy bandgap, planks constant, light frequency and constant (i.e. direct or indirect band-gap), respectively. The band-gap energy has been determined by drawing intercept of the straight line with y-axis for the sample. The band-gap energy of the pure NiFe2O4 nanoparticles is 4.7eV, which is comparable to the already reported value [29].
MWCNTs nanohybrid samples are shown in Fig. 7(a) and (b), respectively. Both patterns confirmed the presence of Ni, Fe and O in desired ratio. The EDS result for nanohybrid in Fig. 7(b) also shows a peak of carbon, which is due to the presence of MWCNTs. The elemental composition of NiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid samples are given in the inset of Fig. 7(a) and (b), respectively. 3.4. UV–vis spectroscopy
4. Dielectric properties The band-gap energy spectra of NiFe2O4 nanoparticles, an essential parameter that determines the electrical conductivity of the materials, are shown in Fig. 8. Tauc's plot has been used to calculate the band-gap energy of the prepared nanoparticles by plotting (αhυ2) versus photon energy, which is derived from UV–Vis absorbance spectra according to the following equation,
(α hU)2 = A(hU − Eg)b
The dielectric constant, ac-conductivity, impedance and Nyquist plot of NiFe2O4 nanoparticles are shown in Fig. 9. The charge storage capacity or polarization is usually related to the dielectric constant. The polarizations in ferrites are induced due to the exchange of Fe3+↔Fe2+ ions, which gives the local displacement of electrons in the direction of applied electric field. The magnitude of exchange depends on the concentration of Fe3+/Fe2+ ion pairs present at octahedral (B) site for
(1)
Fig. 5. FE-SEM images of (a) NiFe2O4 nanoparticles and (b) NiFe2O4/MWCNTs nanohybrid. 4
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Fig. 6. TEM images of NiFe2O4/MWCNTs nanohybrid.
Fig. 7. EDS spectra of (a) NiFe2O4 nanoparticles and (b) NiFe2O4/MWCNTs nanohybrid.
consequential in a decrease of the dielectric constant [32]. The ac-conductivity at 100 Hz is minimum (i.e. 1.88 × 10−6 S/m) and is gradually reached to 2.5 × 10−4 S/m at 5 MHz as shown in Fig. 9 (a). At lower frequencies, the hopping of Fe2+and Fe3+ ions is less because the grain-boundaries are more active at lower frequencies. While on the other side, the conductive grains become more active at higher frequencies hence promoting the hopping between Fe2+and Fe3+ ions, thereby, increasing the conduction. That's why a gradual raise in conductivity was seen with frequency. Nyquist graph was plotted between real and imaginary parts of impedance as shown in Fig. 9(d). Nyquist plot is typically used to find out the bulk and grain-boundary involvement to the total conductivity. The graph showed one partial semi-circle for nickel ferrite nanoparticles, which showed that resistance is only due to the grain-
the given ferrite [30]. Contrary to this, the dielectric constant exhibits an inverse relation with frequency. At lower range frequency, the dielectric real part is decreased and then it remains constant at higher frequencies. The exponential variation in the dielectric real part can be due to space charge polarization. According to Maxwell–Wagner type theory [31], the space charge polarization is usually occurred due to inhomogeneous dielectric configuration of the material. According to this theory, dielectric materials are composed of well conducting large grains that are separated by poorly conducting thin inter-grain boundaries. It was concluded that the main responsible of the electric polarization in these ferrites are electron exchange between Fe2+ and Fe3+ ions in a directional electric field. This electronic exchange between Fe2+ and Fe3+ ions can't follow the alternating field, when the frequency of the externally applied electric field is increased, 5
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and Fe2+ or insertion of Li+ ions into the NiFe2O4 nanoparticles [34]. The 1st discharge cycle showed a maximum initial capacity of 1148 mAh/g for pure NiFe2O4 nanoparticles that is three times larger than the commercial graphite anode materials (i.e. 372 mA/g) and greater than theoretical discharge capacity of NiFe2O4 (i.e. 915 mAh/ g). The surplus irreversible capacity from theoretical capacity is attributed to the smaller particle size and electrolyte decomposition, which cause amorphization of nano-crystalline particles in the low potential region [35]. The stabilized electrochemical window becomes narrow for the nano-sized electrode materials, hence raise the risk of electrolyte decomposition. The solid-electrolyte interface (SEI) formation (i.e. organic passivating layer form on the surface of the subsequent particles) may take place because of this reaction, which consumes a lot of Li + ions supplied by cathode and hence capacity fading in further cycles [36]. At 1st discharge cycle, the large voltage plateau at around 0.8 V followed by a slope up to the cut-off voltage of 0.01 V. This showed the typical characteristics of voltage trend for the transition metals ferrite nano-materials [13,16,33–35,37]. It is associated with the reduction reaction of Ni2+, Fe3+ and Fe2+ into Ni0 and Fe0 due to Li + intercalation [16], expressed by following reaction;
Fig. 8. UV–visible spectra of NiFe2O4 nanoparticles. In the inset, there is shown the bandgap energy of NiFe2O4 nanoparticles.
boundary contribution.
NiFe2 O4 + 8Li+ +8e−
Charging
⇄ Discharging
Ni+2Fe + 4Li2 O
(2)
5. Electrochemical performance The charge/discharge profile after 1st discharge is drastically changed in subsequent cycles and steeper sloping profile of discharge curves becomes dominant at 2nd and 5th discharge cycles as shown in Fig. 10(a). This is attributed to the severe evolution of the site energy upon the structural degradation [38]. The sloping behavior is expected for the insertion/deinsertion of Li+ ions into the surface layers showing capacitor behavior [39]. The capacitor behavior becomes dominant as the crystal size is decreased because the surface area and site energy are increased with particle size reduction. The reduction in particle size increases the capacitor behavior but it also drops off the specific capacity [33,40]. Several plateaus can be observed during the beginning of discharge cycles, which ascribed the Li ions insertion into the nanosized materials, sequential reduction of transition metals and the atom ordering during the NiO reduction [41].
The charge and discharge rate (C-rate) of the fabricated coin cells were measured using theoretical capacity of electro-active material NiFe2O4 i.e. 915 mAh/g equal to 1C. The galvanostatic charging/discharging graph of pure NiFe2O4 nanoparticles at the C-rate of 0.2C (i.e.183 mAh/g) up to 5 cycles is shown in Fig. 10(a). The curves demonstrate the oxidation/reduction reactions during the lithium insertion and extraction. It can be observed that the discharge potential was decreased gradually from OCV (i.e. 2.68 V) to 0.75–0.9 V with a large discharge plateau at 1st discharge cycle and the plateau shift to the high potential of 1–1.3 V at 2nd and 5th discharge cycles. This is attributed to the reduction in particles size during lithium ions intercalation [33]. The 1st charge curve shows the linear slope region from 1.1 V to 2.3 V, which is attributed to the reversible oxidation of Ni0 and Fe0 to Ni2+
Fig. 9. (a) Variation of ac-conductivity versus frequency of NiFe2O4 nanoparticles, (b) Variation of impedance versus frequency of NiFe2O4 nanoparticles, (c) Variation of dielectric constant versus frequency of NiFe2O4nanoparticles, and (d) Nyquisit plot of NiFe2O4 nanoparticles. 6
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Fig. 10. Galvanostatic charging/discharging curve for (a) NiFe2O4 nanoparticles (b) NiFe2O4/MWCNTs nanohybrid.
The charge-discharge curves for the NiFe2O4/MWCNTs nanohybrid are shown in Fig. 10(b). It can be clearly observed that the 1st and 2nd discharge curves gave the capacity of 1305 mAh/g and 1027 mAh/g with columbic efficiency of 79%, respectively. It is an 8% increase in the columbic efficiency from that of pure NiFe2O4 nanoparticles (i.e. 71%). The improved discharge capacity and columbic efficiency of such nanohybrid may be ascribed to the extra Li + ions insertion on the surface of MWCNTs and well distribution nanoparticles on the surfaces of MWCNTs. Approximately same electrochemical potential profile for 2nd and 5th discharge curve in Fig. 10(b) can be observed, which shows the negligible structural distortion as compared to pure NiFe2O4 nanoparticles and is attributed to better interface between MWCNTs and NiFe2O4 nanoparticles. The cyclic stability curves of pure NiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid are shown in Fig. 11. The storage capacity at 1st cycle is 1305 mAh/g, which is reduced to 871 mAh/g at 25th cycle for these nanohybrid that is higher than that of the pure NiFe2O4 nanoparticles (i.e. 1150–828 mAh/g). This enhanced capacity is attributed to the synergistic effect and highly stable interface between MWCNTs and NiFe2O4 nanoparticles because MWCNTs provide the conducting path for easy transportation of Li + ions and electrons during charging/discharging process [10]. The porous structure of nanohybrid may cause increase in the specific capacity to store more Li+ ions at voids. The columbic efficiency remained 92% from 5th cycle up to 25th cycles. The capacity loss of almost 21% between 2nd and 25th cycle is usually due to agglomeration of nanoparticles because of their high surface energy formation at solid-electrolyte interface (SEI) [14].
The depletion of Li-ions will occur across NiFe2O4 nanoparticles because of their high resistivity and low conductivity. Similar cyclic performance can be observed for both samples, which may be caused by low loading ratio of MWCNTs (i.e. 5%). A high loading ratio is essential to achieve high storage capacity as well as good cycling stability [34]. The almost good cyclic stability of pure NiFe2O4 nanoparticles as compared to previous reported studies is ascribed to selective synthesis route, large and uniformly distributed particles sizes (i.e. 25–51 nm) [16,35,41,42]. The increase in particle size reduces the site energy and hence lowers the particles aggregation and improves cyclic stability. The rate capability of pure NiFe2O4 nanoparticles and NiFe2O4/ MWCNTs nanohybrid at different C-rate are shown in Fig. 12 and its values are given in Table 2. The NiFe2O4/MWCNTs nanohybrid shows a remarkably improved rate capability as compared to pure NiFe2O4 nanoparticles. More importantly, after charging/discharging tests at high C-rate, the capacity still returns back to the value of 905 mAh/g at 0.2C rate after 30th cycles giving 97% capacity retention, showing high stability of the prepared NiFe2O4/MWCTs nanohybrid. This can be attributed to the presence of metallic MWCNTs that provides electronically conducting channels for the rapid lithium diffusion. Due to πorbital overlap in metallic MWCNTs, the electron movement can occur via ballistic transport (i.e. electron can transfer with mean free paths of the order of microns along the length of the nanotubes unless scattered by a defect). This kind of property has the potential to increase C-rate performance. The storage capacity at 5C (i.e. 4570 mAh/g) is 634 mAh/ g for NiFe2O4/MWCTs nanohybrid, which is higher than all the previously reported studies for NiFe2O4 nanoparticles-based nano-
Fig. 11. Cyclic stability comparison curves at 0.2C for NiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid.
Fig. 12. Rate capability curves for NiFe2O4 nanoparticles and NiFe2O4/ MWCNTs nanohybrid. 7
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Table 2 Specific storage capacity at different C-rate ofNiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid. C-rate
0.2 0.5 1 2 5
Specific capacity (mAh/g) of NiFe2O4 nanoparticles
Specific capacity (mAh/g) of NiFe2O4/MWCNTs nanohybrid
839 784 715 617 501
958 861 780 722 634
[15]
[16]
[17]
[18]
[19]
composites [16,34,35,43]. This is because of the metallic nature of MWCNTs, which sustain electron movement particularly at higher current rates without any change in structure [10]. This behavior also suggests that the NiFe2O4 nanoparticles have strong bond with MWCNTs via metal-oxygen interaction, which facilitates the fast electron transfer from MWCNTs to NiFe2O4 nanoparticles.
[20]
[21] [22]
6. Conclusion NiFe2O4/MWCNTs nanohybrid material was successfully synthesized by two-step novel method. First, the pure NiFe2O4 nanoparticles were synthesized by co-precipitation route and then embedded on the surface of MWCNTs by ultrasonication assisted method, using toluene as a dispersive and/or linkage medium. This nanohybrid material showed better electrochemical performance (i.e. 1305 mAh/g followed by 871 mAh/g at 25th cycle) and rate capability (i.e. 634 mAh/g at 5C) than other such type of composition used as an anode material for lithium ion battery applications. The better rate capability ascribed to the good interaction between pure NiFe2O4 nanoparticles and MWCNTs, which proved that our opted method for synthesis of NiFe2O4/MWCNTs nanohybrid material was of high efficiency. This simple approach of samples synthesis could be used to prepare several nanohybrids of MFe2O4 (where M = Co, Cu, Mn, Zn etc) nanoparticles and carbonaceous nanomaterials as anode materials with high storage capacities and rate capabilities for next generation Lithium ions batteries.
[23]
[24] [25]
[26] [27]
[28]
[29]
[30]
[31] [32]
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NiFe2O4 nanoparticles/MWCNTs nanohybrid as anode material for lithiumion battery Muhammad Mujahida,b, Rafi Ullah Khana, Muhammad Mumtazb,∗, Mubasherb, Sumair Ahmed Soomroc, Shafiq Ullahd a
Department of Mechanical Engineering, Faculty of Engineering and Technology (FET), International Islamic University (IIU), Islamabad, 44000, Pakistan Materials Research Laboratory, Department of Physics, Faculty of Basic and Applied Sciences (FBAS), International Islamic University (IIU), Islamabad, 44000, Pakistan c National University of Sciences and Technology (NUST), Islamabad, Pakistan d National Engineering and Scientific Commission, Islamabad, Pakistan b
A R T I C LE I N FO
A B S T R A C T
Keywords: NiFe2O4/MWCNTs nanohybrid Co-precipitation Toluene Lithium-ion battery Dielectric properties
Nickel ferrite (NiFe2O4) nanoparticles were first prepared by co-precipitation route and then they were combined with multiwall carbon nanotubes (MWCNTs) using ultra-sonication assisted method. A facile two-step method was used to synthesize NiFe2O4/MWCNTs nanohybrid as an anode material to modify the electrochemical performance of lithium-ion battery. For the first-time, Toluene was used as a dispersive/linkage medium for the synthesis of this nanohybrid material. The formation of inverse spinel cubic structure with no impurity was investigated by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The average particle size of spherical nanoparticles was found about 36 nm NiFe2O4nanoparticles were embedded on the surface of MWCNTs. This nanohybrid material showed high specific capacity (i.e. 1302 mAh/g) and better cyclic performance (i.e. 871 mAh/g) after 25 cycles at 0.2 C-rate. More importantly, this showed excellent rate capability (i.e. 631 mAh/g) even at high 5 C-rate and returned to the initial capacity at 0.2C by giving 97% columbic efficiency.
1. Introduction Due to the increasing consumption and fast depletion of fossils fuels, the focus of the scientists and the researchers has been shifted towards capitalization of renewable energies i.e. wind energy, tidal energy, solar energy and others. To achieve widespread applications of these renewable energies, high energy storage devices and conversion technology is required. Subsequently, lithium-ion batteries attracted more attention because of its design flexibility and high specific energy storage [1,2]. Material of anode plays the key role that influences the net performance of lithium-ion batteries. It is very necessary to manufacture the anode of lithium-ion batteries from robust, non-toxic and economical materials to overcome the rising demand of high energy storage devices. For this purpose, nano-structured transition metal oxides (i.e. Mn3O4, Co3O4, Fe2O3, SnO and SiO2 etc) have been widely used as a promising anode materials [3–7]. Moreover, divalent metalbased ferrites (i.e. MFe2O4 where M = Co2+, Ni2+, Cu2+, Mn2+etc) got more attention because of their low cost, environment friendly and most importantly their high lithium-ion storage capacity (i.e. more than 900 mAh/g), which is much higher than commercially used graphite
∗
anode (i.e. 372 mAh/g). Among these ferrites, NiFe2O4 material is considered as a soft magnetic material having cubic spinel structure with prominent electrochemical properties due to its massive surface atoms percentage, large specific surface area and insulating behavior, which often make it suitable as a charge storage material. The only drawback of these oxides nanoparticles are their low conductivity and agglomeration that caused severe capacity fading during charging/ discharging cycles [8,9]. This problem can be solved by combining these ferrites materials with some other materials having excellent electrical conductivity. Carbonaceous nano-materials such as carbon nanotubes (CNTs) and graphene nano-sheets, have already been proven of having good chemical inertness, superior electrical conductivity and high contact area for utilization in energy storage devices [8]. CNTs have already been widely used by researchers due to its low cost and easy availability. Keeping in mind the properties of both NiFe2O4 and CNTs, it's rather natural to combine both into a nanohybrid with the complimentary properties of both components. CNTs usually provide structure stability when used as a filler material with nano-structured transition metal oxides [10]. Most importantly, the insertion of CNTs prevent the
Corresponding author. E-mail address: [email protected] (M. Mumtaz).
https://doi.org/10.1016/j.ceramint.2019.01.160 Received 21 December 2018; Received in revised form 15 January 2019; Accepted 21 January 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mujahid, M., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.01.160
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2.2. Samples characterization
aggregation, expansion and contraction in volume of the nanoparticles of the electrodes during discharging and charging process [8,11]. It also works as a highly conductive matrix and creates a contact between electrode and current collector. Meanwhile, the decoration of transition metal oxides nanoparticles on the surface of the CNTs reduces the restacking of nanotubes sustaining the high surface area [12]. Due to synergistic effects by combining the high capacity transition-metal oxides nanoparticles with high surface area/conductivity of CNTs, the improvement in cyclic stability, rate capability and storage capacity in CNTs based nanohybrids for lithium-ion batteries has been observed [11,13–18]. Researchers opted various routes to synthesize the nanoparticles/ CNTs nanohybrids [15,19–23]. The previous studies primarily focused on embedding nanoparticles on CNTs by using xylene, pyrene, benzyl alcohol and phenylphosphoric acid as derivative linkages [23–25]. Toluene has been used for the first time as dispersive medium in the current research work. Also, the use of MWCNTs with ferrite nanoparticles to synthesize nanohybrids has not yet been studied for lithium ion batteries. In literature, single wall carbon nanotubes (SWCNTs) have already been used for such types of nanohybrids. But the drawbacks of SWCNTs are that they are very expensive and their nanohybrids have shown less rate performance. The semiconducting nature of SWCNTs leads to slow kinetics, which can be responsible for poor rate capability, while MWCNTs are considered to be metallic in nature and thus are expected to offer better rate capability, as compared to SWCNTs [10]. That's why, the main focus of this study is to improve the electrochemical performance, most preferably rate capability as well as storage capacity. We preferred to use Toluene as dispersive medium because it has aromatic ring structure as that of xylene, with single methyl-group, π-π interaction mechanism and less density than xylene [24,25]. In this work, NiFe2O4 nanoparticles were first synthesized by co-precipitation method and then embedded on the surface of MWCNTs by sonication assisted method using Toluene as a dispersive/linkage medium to modify the storage capacity, cyclic stability and rate capability.
The crystal structure and phase formation of NiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid in powder form were investigated by Bruker D8 Advance X-rays Diffraction (XRD) equipment using CuKα (λ = 1.5405 Å) radiation. Fourier transform infrared (FTIR) spectroscopy of NiFe2O4 nanoparticles, MWCNTs and NiFe2O4/MWCNTs nanohybrid were carried out in the range from 400 to 4000 by using KBr pellets. The morphology of these nanoparticles and nanohybrid were examined by scanning electron microscope (SEM) (Model JEOL-JSM6390 and Vega3 Tescan), field emission scanning electron microscope (FE-SEM) (Model FEI Panalytical Quanta 450) and transmission electron microscope (TEM). The band-gap energy of the samples was measured by UV–visible spectroscopy (Perkin-Elmer equipment). The ac-conduction properties were performed on the pellets of these samples sintered at 450 °C for 2 h by using impedance analyzer (LCR 6500B WK model). The thermo-gravimetric analysis (TGA) was carried out at ambient temperature to 1000 °C by using NETZSCH Jupitor, model STA 449-F3 equipment. The chemical composition of the samples were determined by energy dispersive spectroscopy (EDS). 2.3. Battery testing The electrodes for Li-ions battery were fabricated of thick slurry of active material (NiFe2O4 nanoparticles, NiFe2O4/MWCNTs nanohybrid), binder polyvinylidene fluoride (PVDF) and conducting carbon in the weight ratio 8:1:1.1-methyl 2-pyrolidone (NMP) solvent was used to mix slurry completely. The thick and homogeneous slurry was coated uniformly on one side of a copper foil and heated at 100 °C for 12 h. The dried thick films of electrodes were pressed at about 1500 kPa pressure and cut into circular discs of 16 mm diameter for good adherence of electrode material on the copper foil. Pure lithium metal foil was used as counter electrode, polyethylene was used as a separator and 1 M LiPF6 solution dissolved in ethylene carbonate dimethyl carbonate (1:1) was used as electrolyte medium. These electrodes were assembled in argon containing glove box in the form of CR-2023 type coin-cell.
2. Experimental details 3. Results and discussion 2.1. Preparation of NiFe2O4/MWCNTS nanohybrid 3.1. XRD and TGA analysis Typically, the salts of metals in the form of nitrate (Sigma-Aldrich) and sodium hydroxide were used as precursors for the preparation of nano-crystalline NiFe2O4 particles by co-precipitation route. Initially, 0.1 M solution of nickel nitrate hexahydrate (i.e. Ni(NO3)2.6H2O) and 0.2 M solution of iron (III) nitrate nonahydrate (i.e. Fe(NO3)2.9H2O) were mixed with each other by stirring. The mixture was then heated at 85–90 °C and a 3 M solution of sodium hydroxide previously heated at 80 °C was added and constantly stirred for an hour at 90 °C. The product was cooled down to room temperature and washed with double distilled water several times until the pH 7 was achieved. The resultant solution was dried by overnight heating at 100 °C followed by annealing at 800 °C for 8 h to get the distinct impurity free spinel phase structure of NiFe2O4 nanoparticles. The powder was ground in mortar and pestle to eliminate agglomeration and to obtain fine nanostructures. These NiFe2O4 nanoparticles were added into 50 ml of Toluene separately and sonicated for an hour at room temperature for uniform dispersion. After the uniform dispersion of these NiFe2O4 nanoparticles in Toluene medium, 5% of MWCNTs were added in the medium. The mixture was again sonicated for 5 h using water bath sonicator. Toluene fluid was evaporated by keeping it at 100 °C leaving behind uniformly dispersed NiFe2O4/MWCNTs nanohybrid. This nanohybrid material was further homogenized using mechanical mixing (solid-state method) to avoid agglomeration. Finally the prepared NiFe2O4/MWCNTs nanohybrid was calcined at 350 °C for 2 h for the good adhesion of NiFe2O4 nanoparticles with MWCNTs walls.
The powder XRD technique was used to investigate the crystal structure and phase formation of the samples. The XRD profile of pure NiFe2O4 nanoparticles, MWCNTs and NiFe2O4/MWCNTs nanohybrid are shown in Fig. 1. The peaks (111), (220), (311), (400), (422), (511),
Fig. 1. XRD spectra of (a) MWCNTs, (b) NiFe2O4 nanoparticles and (c) NiFe2O4/MWCNTs nanohybrid. 2
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(440) and (533) confirmed the expected closed packed FCC cubic structure with Fd3m space group of NiFe2O4 nanoparticles in accordance with card number JCPDS 22–1086. The distinct appearance of peak (311) at 35.760° showed good crystallinity of the product while the FWHM showed the formation of small crystallite size, which in turn confirmed the formation of small size grains. No additional peak appeared other than that of cubic arrangement in the pattern, which showed that no impurity was present in the product material. The crystallite size calculated by Debye-Scherrer's formula from the XRD data of NiFe2O4 nanoparticles was 29 nm. In XRD spectrum of MWCNTs, the characteristics (002) and (100) peaks represented the graphitic reflection. The XRD spectrum of NiFe2O4/MWCNTs nanohybrid gave almost the same spectrum as that of pure NiFe2O4 nanoparticles and graphitic peaks was not found, which accounted for the complete anchoring of NiFe2O4 nanoparticles on the surface of MWCNTs, indicating the uniform dispersion and the effectiveness of the opted method. However, the broadening of the peaks on coating showed the decrease in crystallite size (i.e. 24 nm) and ascribed to adsorption of nanoparticles on the surface of MWCNTs, which diminished agglomeration and thus crystallite size. The bulk density ‘Db’ (i.e. the ratio between mass and bulk volume), the X-ray density ‘Dx’ (i.e. measured from XRD data) and the porosity fraction (i.e. 1 – (Db/Dx)) measured for pure NiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid are given in Table 1. Since MWCNTs were porous in nature with heptagon-pentagon pair and interstitial defects, the decrease in density and increase in porosity were expected for prepared nanohybrid. Thermo-gravimetric analysis (TGA) were performed for both the samples from room temperature to 1000 °C at a rate of 5 °C/min to quantify the volume of MWCNTs and NiFe2O4 nanoparticles in nanohybrid as shown in Fig. 2. The percentage loss of a pure NiFe2O4 nanoparticles was just 2%, while the nanohybrid showed rapid mass loss upto 9% at the range of 700 °C–900 °C, which is attributed to the oxidation of MWCNTs. Hence the amounts of the NiFe2O4 nanoparticles in NiFe2O4/MWCNTs nanohybrid are about 91% as calculated from the residual weight loss.
Fig. 2. TGA pattern of prepared samples.
Fig. 3. FTIR spectra of (a) MWCNTs (b) NiFe2O4 nanoparticles and (c) NiFe2O4/ MWCNTs nanohybrid.
3.2. FTIR spectroscopy The FTIR spectra of pure NiFe2O4 nanoparticles, MWCNTs and NiFe2O4/MWCNTs nanohybrid obtained at room temperature is shown in Fig. 3. In case of pure NiFe2O4 nanoparticles, two strong absorption bands v1 around 597 cm−1 and v2 around 408 cm−1 represents tetrahedral (A) and octahedral (B) sites, respectively [23]. These ν1 and ν2 absorption bands are in fact credited to the metal-oxygen stretching vibration mode at A and B sites. The formation of these two sub-lattices bands in the FTIR spectra confirmed the formation of single-phase spinel structure and compatible with the previously reported results [26]. In the FTIR spectra of MWCNTs, the stretching mode at 1628 cm−1 is assigned to the vibration of C = C graphitic layers, which was formed due to MWCNTs sidewall framework [27]. The broad peak center at 3439 cm−1 corresponds to the eOH (hydroxyl) stretching band [28]. In case of NiFe2O4/MWCNTs nanohybrid, a shift of bands is observed, which indicates the subsistence of inter-molecular forces between MWCNTs and NiFe2O4 nanoparticles.
3.3. Morphology and chemical analysis The structural and chemical analysis of the materials was performed by using SEM, FE-SEM, TEM and EDS. The SEM, FE-SEM and TEM images of MWCNTs, pure NiFe2O4 nanoparticles, and NiFe2O4/ MWCNTs nanohybrid at different magnification are shown in Fig. 4(a–d), Fig. 5(a and b) and Fig. 6, respectively. It can be clearly observed that pure NiFe2O4 nanoparticles are spherical in shape with average diameter of 36 nm that is in good agreement with the calculated size by Debye-Scherrer's formula. A small agglomeration can also be observed, which can be accounted for the annealing of the synthesized product owing to the diffusion of the atoms and high surface energy. However, when particles normally come into contact with each other and under favorable energy conditions, some of them grow while others decrease in size and eventually disappear. So, as a result a small number of large particles are evident from these images. The crystallite clusters cemented together can be easily visible from the FE-SEM images as shown in Fig. 5(a and b). The diameter of free MWCNTs is measured as 25–35 nm. The SEMs of NiFe2O4/MWCNTs nanohybrid without grinding and sonication and after grinding and sonication are shown in Fig. 4(c) and (d), respectively. The large numbers of nanoparticles are completely embedded on the wall surfaces of MWCNTs even after grinding as shown in Figs. 4(d), 5(b) and 6, due to π-π interaction [24,25], which confirm that our opted dispersion procedure for preparation of the nanohybrid was of high efficiency. The EDS analysis patterns of NiFe2O4nanoparticles and NiFe2O4/
Table 1 Variation of X-ray density, bulk density, volume of cell, crystallite size and porosity of NiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid. Parameters 3
X-ray Density (g/cm ) Bulk Density (g/cm3) Volume of cell (nm3) Crystallite Size (nm) Porosity (%)
NiFe2O4 nanoparticles
NiFe2O4/MWCNTs nanohybrid
5.30 3.33 579.51 29 37.2
5.16 2.49 581.62 24.5 43.6
3
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Fig. 4. SEMs of (a) NiFe2O4 nanoparticles (b) MWCNTs (c) NiFe2O4/MWCNTs nanohybrid without grinding and sonication (d) NiFe2O4/MWCNTs nanohybrid after grinding and sonication in distilled water.
where α, Eg, h, υ and b represent absorption coefficient, energy bandgap, planks constant, light frequency and constant (i.e. direct or indirect band-gap), respectively. The band-gap energy has been determined by drawing intercept of the straight line with y-axis for the sample. The band-gap energy of the pure NiFe2O4 nanoparticles is 4.7eV, which is comparable to the already reported value [29].
MWCNTs nanohybrid samples are shown in Fig. 7(a) and (b), respectively. Both patterns confirmed the presence of Ni, Fe and O in desired ratio. The EDS result for nanohybrid in Fig. 7(b) also shows a peak of carbon, which is due to the presence of MWCNTs. The elemental composition of NiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid samples are given in the inset of Fig. 7(a) and (b), respectively. 3.4. UV–vis spectroscopy
4. Dielectric properties The band-gap energy spectra of NiFe2O4 nanoparticles, an essential parameter that determines the electrical conductivity of the materials, are shown in Fig. 8. Tauc's plot has been used to calculate the band-gap energy of the prepared nanoparticles by plotting (αhυ2) versus photon energy, which is derived from UV–Vis absorbance spectra according to the following equation,
(α hU)2 = A(hU − Eg)b
The dielectric constant, ac-conductivity, impedance and Nyquist plot of NiFe2O4 nanoparticles are shown in Fig. 9. The charge storage capacity or polarization is usually related to the dielectric constant. The polarizations in ferrites are induced due to the exchange of Fe3+↔Fe2+ ions, which gives the local displacement of electrons in the direction of applied electric field. The magnitude of exchange depends on the concentration of Fe3+/Fe2+ ion pairs present at octahedral (B) site for
(1)
Fig. 5. FE-SEM images of (a) NiFe2O4 nanoparticles and (b) NiFe2O4/MWCNTs nanohybrid. 4
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Fig. 6. TEM images of NiFe2O4/MWCNTs nanohybrid.
Fig. 7. EDS spectra of (a) NiFe2O4 nanoparticles and (b) NiFe2O4/MWCNTs nanohybrid.
consequential in a decrease of the dielectric constant [32]. The ac-conductivity at 100 Hz is minimum (i.e. 1.88 × 10−6 S/m) and is gradually reached to 2.5 × 10−4 S/m at 5 MHz as shown in Fig. 9 (a). At lower frequencies, the hopping of Fe2+and Fe3+ ions is less because the grain-boundaries are more active at lower frequencies. While on the other side, the conductive grains become more active at higher frequencies hence promoting the hopping between Fe2+and Fe3+ ions, thereby, increasing the conduction. That's why a gradual raise in conductivity was seen with frequency. Nyquist graph was plotted between real and imaginary parts of impedance as shown in Fig. 9(d). Nyquist plot is typically used to find out the bulk and grain-boundary involvement to the total conductivity. The graph showed one partial semi-circle for nickel ferrite nanoparticles, which showed that resistance is only due to the grain-
the given ferrite [30]. Contrary to this, the dielectric constant exhibits an inverse relation with frequency. At lower range frequency, the dielectric real part is decreased and then it remains constant at higher frequencies. The exponential variation in the dielectric real part can be due to space charge polarization. According to Maxwell–Wagner type theory [31], the space charge polarization is usually occurred due to inhomogeneous dielectric configuration of the material. According to this theory, dielectric materials are composed of well conducting large grains that are separated by poorly conducting thin inter-grain boundaries. It was concluded that the main responsible of the electric polarization in these ferrites are electron exchange between Fe2+ and Fe3+ ions in a directional electric field. This electronic exchange between Fe2+ and Fe3+ ions can't follow the alternating field, when the frequency of the externally applied electric field is increased, 5
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and Fe2+ or insertion of Li+ ions into the NiFe2O4 nanoparticles [34]. The 1st discharge cycle showed a maximum initial capacity of 1148 mAh/g for pure NiFe2O4 nanoparticles that is three times larger than the commercial graphite anode materials (i.e. 372 mA/g) and greater than theoretical discharge capacity of NiFe2O4 (i.e. 915 mAh/ g). The surplus irreversible capacity from theoretical capacity is attributed to the smaller particle size and electrolyte decomposition, which cause amorphization of nano-crystalline particles in the low potential region [35]. The stabilized electrochemical window becomes narrow for the nano-sized electrode materials, hence raise the risk of electrolyte decomposition. The solid-electrolyte interface (SEI) formation (i.e. organic passivating layer form on the surface of the subsequent particles) may take place because of this reaction, which consumes a lot of Li + ions supplied by cathode and hence capacity fading in further cycles [36]. At 1st discharge cycle, the large voltage plateau at around 0.8 V followed by a slope up to the cut-off voltage of 0.01 V. This showed the typical characteristics of voltage trend for the transition metals ferrite nano-materials [13,16,33–35,37]. It is associated with the reduction reaction of Ni2+, Fe3+ and Fe2+ into Ni0 and Fe0 due to Li + intercalation [16], expressed by following reaction;
Fig. 8. UV–visible spectra of NiFe2O4 nanoparticles. In the inset, there is shown the bandgap energy of NiFe2O4 nanoparticles.
boundary contribution.
NiFe2 O4 + 8Li+ +8e−
Charging
⇄ Discharging
Ni+2Fe + 4Li2 O
(2)
5. Electrochemical performance The charge/discharge profile after 1st discharge is drastically changed in subsequent cycles and steeper sloping profile of discharge curves becomes dominant at 2nd and 5th discharge cycles as shown in Fig. 10(a). This is attributed to the severe evolution of the site energy upon the structural degradation [38]. The sloping behavior is expected for the insertion/deinsertion of Li+ ions into the surface layers showing capacitor behavior [39]. The capacitor behavior becomes dominant as the crystal size is decreased because the surface area and site energy are increased with particle size reduction. The reduction in particle size increases the capacitor behavior but it also drops off the specific capacity [33,40]. Several plateaus can be observed during the beginning of discharge cycles, which ascribed the Li ions insertion into the nanosized materials, sequential reduction of transition metals and the atom ordering during the NiO reduction [41].
The charge and discharge rate (C-rate) of the fabricated coin cells were measured using theoretical capacity of electro-active material NiFe2O4 i.e. 915 mAh/g equal to 1C. The galvanostatic charging/discharging graph of pure NiFe2O4 nanoparticles at the C-rate of 0.2C (i.e.183 mAh/g) up to 5 cycles is shown in Fig. 10(a). The curves demonstrate the oxidation/reduction reactions during the lithium insertion and extraction. It can be observed that the discharge potential was decreased gradually from OCV (i.e. 2.68 V) to 0.75–0.9 V with a large discharge plateau at 1st discharge cycle and the plateau shift to the high potential of 1–1.3 V at 2nd and 5th discharge cycles. This is attributed to the reduction in particles size during lithium ions intercalation [33]. The 1st charge curve shows the linear slope region from 1.1 V to 2.3 V, which is attributed to the reversible oxidation of Ni0 and Fe0 to Ni2+
Fig. 9. (a) Variation of ac-conductivity versus frequency of NiFe2O4 nanoparticles, (b) Variation of impedance versus frequency of NiFe2O4 nanoparticles, (c) Variation of dielectric constant versus frequency of NiFe2O4nanoparticles, and (d) Nyquisit plot of NiFe2O4 nanoparticles. 6
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Fig. 10. Galvanostatic charging/discharging curve for (a) NiFe2O4 nanoparticles (b) NiFe2O4/MWCNTs nanohybrid.
The charge-discharge curves for the NiFe2O4/MWCNTs nanohybrid are shown in Fig. 10(b). It can be clearly observed that the 1st and 2nd discharge curves gave the capacity of 1305 mAh/g and 1027 mAh/g with columbic efficiency of 79%, respectively. It is an 8% increase in the columbic efficiency from that of pure NiFe2O4 nanoparticles (i.e. 71%). The improved discharge capacity and columbic efficiency of such nanohybrid may be ascribed to the extra Li + ions insertion on the surface of MWCNTs and well distribution nanoparticles on the surfaces of MWCNTs. Approximately same electrochemical potential profile for 2nd and 5th discharge curve in Fig. 10(b) can be observed, which shows the negligible structural distortion as compared to pure NiFe2O4 nanoparticles and is attributed to better interface between MWCNTs and NiFe2O4 nanoparticles. The cyclic stability curves of pure NiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid are shown in Fig. 11. The storage capacity at 1st cycle is 1305 mAh/g, which is reduced to 871 mAh/g at 25th cycle for these nanohybrid that is higher than that of the pure NiFe2O4 nanoparticles (i.e. 1150–828 mAh/g). This enhanced capacity is attributed to the synergistic effect and highly stable interface between MWCNTs and NiFe2O4 nanoparticles because MWCNTs provide the conducting path for easy transportation of Li + ions and electrons during charging/discharging process [10]. The porous structure of nanohybrid may cause increase in the specific capacity to store more Li+ ions at voids. The columbic efficiency remained 92% from 5th cycle up to 25th cycles. The capacity loss of almost 21% between 2nd and 25th cycle is usually due to agglomeration of nanoparticles because of their high surface energy formation at solid-electrolyte interface (SEI) [14].
The depletion of Li-ions will occur across NiFe2O4 nanoparticles because of their high resistivity and low conductivity. Similar cyclic performance can be observed for both samples, which may be caused by low loading ratio of MWCNTs (i.e. 5%). A high loading ratio is essential to achieve high storage capacity as well as good cycling stability [34]. The almost good cyclic stability of pure NiFe2O4 nanoparticles as compared to previous reported studies is ascribed to selective synthesis route, large and uniformly distributed particles sizes (i.e. 25–51 nm) [16,35,41,42]. The increase in particle size reduces the site energy and hence lowers the particles aggregation and improves cyclic stability. The rate capability of pure NiFe2O4 nanoparticles and NiFe2O4/ MWCNTs nanohybrid at different C-rate are shown in Fig. 12 and its values are given in Table 2. The NiFe2O4/MWCNTs nanohybrid shows a remarkably improved rate capability as compared to pure NiFe2O4 nanoparticles. More importantly, after charging/discharging tests at high C-rate, the capacity still returns back to the value of 905 mAh/g at 0.2C rate after 30th cycles giving 97% capacity retention, showing high stability of the prepared NiFe2O4/MWCTs nanohybrid. This can be attributed to the presence of metallic MWCNTs that provides electronically conducting channels for the rapid lithium diffusion. Due to πorbital overlap in metallic MWCNTs, the electron movement can occur via ballistic transport (i.e. electron can transfer with mean free paths of the order of microns along the length of the nanotubes unless scattered by a defect). This kind of property has the potential to increase C-rate performance. The storage capacity at 5C (i.e. 4570 mAh/g) is 634 mAh/ g for NiFe2O4/MWCTs nanohybrid, which is higher than all the previously reported studies for NiFe2O4 nanoparticles-based nano-
Fig. 11. Cyclic stability comparison curves at 0.2C for NiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid.
Fig. 12. Rate capability curves for NiFe2O4 nanoparticles and NiFe2O4/ MWCNTs nanohybrid. 7
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Table 2 Specific storage capacity at different C-rate ofNiFe2O4 nanoparticles and NiFe2O4/MWCNTs nanohybrid. C-rate
0.2 0.5 1 2 5
Specific capacity (mAh/g) of NiFe2O4 nanoparticles
Specific capacity (mAh/g) of NiFe2O4/MWCNTs nanohybrid
839 784 715 617 501
958 861 780 722 634
[15]
[16]
[17]
[18]
[19]
composites [16,34,35,43]. This is because of the metallic nature of MWCNTs, which sustain electron movement particularly at higher current rates without any change in structure [10]. This behavior also suggests that the NiFe2O4 nanoparticles have strong bond with MWCNTs via metal-oxygen interaction, which facilitates the fast electron transfer from MWCNTs to NiFe2O4 nanoparticles.
[20]
[21] [22]
6. Conclusion NiFe2O4/MWCNTs nanohybrid material was successfully synthesized by two-step novel method. First, the pure NiFe2O4 nanoparticles were synthesized by co-precipitation route and then embedded on the surface of MWCNTs by ultrasonication assisted method, using toluene as a dispersive and/or linkage medium. This nanohybrid material showed better electrochemical performance (i.e. 1305 mAh/g followed by 871 mAh/g at 25th cycle) and rate capability (i.e. 634 mAh/g at 5C) than other such type of composition used as an anode material for lithium ion battery applications. The better rate capability ascribed to the good interaction between pure NiFe2O4 nanoparticles and MWCNTs, which proved that our opted method for synthesis of NiFe2O4/MWCNTs nanohybrid material was of high efficiency. This simple approach of samples synthesis could be used to prepare several nanohybrids of MFe2O4 (where M = Co, Cu, Mn, Zn etc) nanoparticles and carbonaceous nanomaterials as anode materials with high storage capacities and rate capabilities for next generation Lithium ions batteries.
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