Doping and Surface Modification Enhance the Applicability of Nanostructured Fullerene–MWCNT Hybrid Draped LiNi0.1Mg0.1Co0.8O2 as High Efficient Cathode Material for Lithium-Ion Batteries

Development of high performance cathode materials, layer-structured ternary LiNixCoyM1−x−yO2 cathode materials have attracted much attention owing to their larger capacity and higher energy density. Persistent efforts have been devoted to tackling certain issues like low electronic conductivity and poor structural stability. Dual strategy of Mg doping and surface modification of the cathode material was adopted to improve the performance of the battery. Fullerene–Multi-Walled Carbon Nanotube (MWCNT) hybrid draped LiNi0.1Mg0.1Co0.8O2 nanocomposite was synthesized by a simple chemical route. The fullerene–MWCNT hybrid modifies the surface of pristine LiNi0.1Mg0.1Co0.8O2 thereby improves the electrochemical performance and maintains the structural stability of the cathode material. Pristine LiNi0.1Mg0.1Co0.8O2 and LiNi0.1Mg0.1Co0.8O2/fullerene–MWCNT nanocomposite were studied using various advanced characterization techniques such as X-ray diffraction (XRD), Micro-Raman spectroscopy, Field Emission Scanning Electron Microscopy (FESEM), X-ray Photoelectron Spectroscopy (XPS), and High-Resolution Transmission Electron Microscopy (HRTEM). It is found that LiNi0.1Mg0.1Co0.8O2 particles retain their structural integrity after being enveloped with a fullerene–MWCNT hybrid. The electrochemical performance was investigated with cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) test and electrochemical impedance spectroscopy (EIS). As prepared LiNi0.1Mg0.1Co0.8O2, when deployed in the form of LiNi0.1Mg0.1Co0.8O2/fullerene–MWCNT composite exhibits a high specific capacity of 208 mAh g−1. Fullerene–MWCNT hybrid draped LiNi0.1Mg0.1Co0.8O2 nanocomposite provides an effective Li+ and electron channel that significantly increased the Li-ion diffusion coefficient and reduced the charge transfer resistance. Besides,the lithium diffusion coefficient increased from 5.13 × 10–13 (Li/LiNi0.1Mg0.1Co0.8O2) to 8.313 × 10–13 cm2 s−1 due to the improved kinetics of Li insertion/extraction process in Li/LiNi0.1Mg0.1Co0.8O2 + fullerene–MWCNT cell.


Introduction
Lithium-ion batteries (LIBs) with transitional metal oxide as cathode materials recently play a significant role in energy storage systems for portable electronic devices [1]. There have been several investigations into ground-breaking novel nano-architectures to enhance the efficiency and lifetime of the LIB [2]. Various carbonaceous materials have been tested as electrodes for battery applications. However, to improve the electrochemical performance of layered cathode material, two strategies were often employed (i) partially replacing foreign atoms, such as Cr, Mg, Al, and F, in an attempt to improve structural stability and (ii) surface coating with a thin layer of metal oxide to suppress the lateral effects. In general, the incorporation of carbonaceous materials into transition metal oxide cathode materials to form a conductive network has become a favorable strategy for achieving better cycling performance and rate capability [3]. Owing to their unique physicochemical properties, the nanocarbon materials with different geometrical orientations (e.g., graphene, carbon nanotubes (CNTs), and fullerenes) have gained a lot of applications in lithium-ion batteries as the carbon materials provide enough space for storing lithium ions. The excellent conductivity of the nano carbon material provides excellent lithium storage capacities and rate performances.
Carbon nanotubes are carbon allotropes with a cylindrical shape at the nanoscale level [4]. The structural, electrical, mechanical, and magnetic properties of CNTs, made it a possible candidate for industrial and large-scale applications [4,5]. Exceedingly, CNTs have been employed as conductive nanostructured carbon matrices to boost the specific power, rate capability, and cycling stability of the cathode materials [3]. Other important features that make doped CNTs to be used in lithium-ion batteries are high chemical resistance and low flammability [2]. The electrochemically reversible penetration of Li-ions in SWCNTs and MWCNTs has been extensively studied. Lithium ions diffuse to stable sites on either the outer or the inner surface of a single layer of CNT graphene.
Among various strategies, the bandgap approach has led several research groups, and in particular, Koh et al. [6] reported that the new hybridized SWCNT/C 60 improves the passage of electrons to particle boundaries. Carbon-based materials are promising candidates for constructing 3D conductive networks, which boost active material utilization at high rates during the electrochemical reaction through electrical and ionic pathways [7]. Moreover, as reported in the literature the Li 2 Mn 2.9 Ni 0.9 Co 0.2 O 8 -MWCNT exhibited remarkable interesting properties such as high specific capacity, sufficient rate performance, significant Coulombic efficiency, and improved cycling stability compared with pristine LiNi 0.1 Mg 0.1 Co 0.8 O 2 [8].
Tsuyohiko Fujigaya reported that among the inorganic and polymeric materials, single-walled carbon nanotubes (SWCNT) are particularly attractive in the thermo electric conversion technology due to their non-toxicity, material abundance, solution processability and excellent electrical conductivity [9].
Papageorgiou stated that the two dimensional nature of graphene related materials display a larger surface to volume ratio, which leads to the formation of larger interfaces and production of stronger composites [10]. Samad et.al highlighted the unique characteristics of CNT such as (i) high surface area (1315 m 2 g −1 ) (ii) less impurity (iii) high electrical conductivity (iv) free from deep crack and (v) high catalytic activity due to excellent interaction between catalytic metals and CNT supports,which makes CNT a best catalyst material for energy conversion fuel cell [11][12][13]. Zhang et al. reported that the carbon surface-treated C-LMNCO electrode material for lithium-ion batteries exhibited better cyclic behaviour, lower irreversible capacity loss and acceptable rate discharge capacities [14]. Ma et al. showed that the graphene oxide-coated SnO 2 -Li 1/3 Co 1/3 Mn 1/3 O 2 (GO-SnO2-NCM) cathode material synthesized via a facile wet chemical method exhibited excellent cycling performance, with 91% capacity retention at 1C after 100 cycles, which was higher than 74% for the pristine NCM at the same cycle [15].
Our work broadens the scope of cathodic materials based on LiNi 0.1 Mg 0.1 Co 0.8 O 2 . In fact, Zhang [16] have shown that the layered cathodic material like LiNi 0.65 Mg 0.05 Co 0.3 O 2, synthesized by the sol-gel method delivered an initial discharge capacity of 178 mAh g −1 due to fewer cations mixtures. Xiang et al. [17] stated that the Mg-doped LiNi 0.8 Co 0.2 O 2 electrode delivered an initial discharge capacity of 188 mAh g −1 and retained good cycling behavior. Cho and Park [18] suggested that the LiNi 0.74 Co 0.2 Mg 0.06 O 2 electrodes delivered a discharge capacity of 158 mAh g −1 with improved thermal stability.
Therefore, the electrochemically inactive doping element Mg in LiNi x Co y O 2 elevates the capacity of the cathodic material and retains high structural stability. The stability of cathode materials is enhanced by the promotion of lithiumion transport due to the increased lithium slab gap by the Mg effect [14]. In addition, Mg 2+ ions migrate to the interslab sites that were originally occupied by lithium ions, act as pillars, and sustain the delithiation structures, suppressing the cationic migration [19]. Many techniques have been explored to maintain the surface integrity of the cathode material, in which surface alteration is one of the simplest and most realistic methods that can effectively overwhelm the growth of solid electrolyte interphase level.
In this study, to explore the constructive outcomes such as improved cycling and electrochemical performance of the cathode material, LiNi 0.1 Mg 0.1 Co 0.8 O 2 cathode material was synthesized by self-sustaining combustion route and LiNi 0.1 Mg 0.1 Co 0.8 O 2 /fullerene-MWCNT nanocomposite was synthesized by a simple chemical route. The novelty of the work lies in the addition of nickel to a very small percentage of the composite cathode, with Mg as a dopant, and the incorporation of fullerene-MWCNT hybrid into the layered cathode material. The partial substitution of cobalt with magnesium had a positive effect, minimized cation mixing, and favored the formation of layered cathode structures.
To the best of our knowledge, the influence of fullerene-MWCNT hybrid modification on the characteristics of the LiNi 0.1 Mg 0.1 Co 0.8 O 2 with a very low content of nickel has not been reported yet. In this regard, the current piece of work assumes significant importance, as one of the early works demonstrates that for the first time

Instruments Used for Structural Characterization
The structural properties were studied by X-ray diffractometer (XRD, Shimadzu XRD-6000) and the morphological properties of as-prepared samples were analyzed by fieldemission scanning electron microscopy (FESEM, JSM-6700F 15 kV), and high-resolution transmission electron microscopy (HRTEM, JEOL 2010 200 kV). Micro-Raman scattering measurements were conducted using a DXR Raman Microscope (Thermo Fisher Scientific Inc.) and the source of excitation used was an argon-ion laser operating at 514 nm with a laser incident power of 0.3 mW. For quantitative chemical insights, the as-synthesized particles were characterized by X-ray photoelectron spectroscopy (XPS).

Fabrication of Electrode and Electrochemical Measurements
The synthesized LiNi 0.1 Mg 0.1 Co 0.8 O 2 /fullerene-MWCNT composite material, acetylene black and polyvinylidene fluoride (PVDF) in the proportion of 80:10:10 in N-Methyl-2-Pyrrolidone (NMP) were added together and mixed thoroughly to obtain the slurry. The slurry was carefully coated onto an aluminium foil by the doctor-blade technique and dried at 70 °C to remove water content. The 2032-type coin cells with the coated material as cathode, lithium metal as an anode, and Celgard 2325 membrane as the separator were assembled. In a similar way, the 2032-type coin cells with LiNi 0.1 Mg 0.1 Co 0.8 O 2 coated material as the cathode, lithium metal as an anode, and Celgard 2325 membrane as the separator were also assembled. The separator was wetted with the electrolyte 1M LiPF 6 in EC: DEC; 1:1(v/v). Between a potential window 2.5-4.5V, the cycling profile for the prepared coin cells was studied at room temperature. The voltammograms were analysed using the SP150 (Bio-logic) electrochemical impedance analyser at 0.1 mV s −1 . Electrochemical Impedance Spectroscopy (EIS) for coin cells was recorded between 200 kHz and 100 mHz frequency. nanocomposite were confirmed by the existence of diffraction peak at 2θ = 38° and 46°. A well-layered nanostructure was observed from the distinct splitting of 006/102 and 108/110 doublet peaks [20].

XRD Analysis
The XRD pattern of the fullerene-MWCNT hybrid, shown in Fig. 2c which displayed two peaks located at 2θ = 25.74° & 42.87° can be ascribed to the hexagonal graphite crystal planes of (002) and (001) respectively. Also, the XRD pattern of fullerene-MWCNT hybrid draped LiNi 0.1 Mg 0.1 Co 0.8 O 2 nanocomposite does not reveal the diffraction peaks of graphene or graphite is likely due to the extremely crystalline phase of LiNi 0.1 Mg 0.1 Co 0.8 O 2 nanocomposite [5].
The nanocomposite was found to be higher than the 4.9. Also, the ratio of I 003 /I 104 for the pristine and fullerene-MWCNT hybrid draped LiNi 0.1 Mg 0.1 Co 0.8 O 2 composite is > 1.2 indicating the low (Li + and Ni 2+ ) cation mixing [16].
The physical mixing of fullerene-MWCNT hybrid in LiNi 0.1 Mg 0.1 Co 0.8 O 2 has been reported to have an insignificant effect on the properties of MWCNTs relative to the covalent hybrid structure. The XRD results show that the surface modification with fullerene-MWCNT hybrid does not affect the crystal structure and preserves the structural stability of the electrode component.

FESEM Analysis
The sur face mor phology patter ns of pr istine LiNi 0.1 Mg 0.1 Co 0.8 O 2 , fullerene-MWCNT hybrid, and fullerene-MWCNT hybrid draped LiNi 0.1 Mg 0.1 Co 0.8 O 2 nanocomposite electrode were examined by FESEM as shown in Fig. 3 respectively. Incidentally, Fig. 3a shows the formation of cubic shaped like nanoparticles of pristine LiNi 0.1 Mg 0.1 Co 0.8 O 2 which is also confirmed from the high-resolution TEM images as shown in Fig. 5a. The FESEM images as shown in Fig. 3b of fullerene-MWCNT hybrid show small spherical shaped like fullerene nanoparticles with an average particle size of 22 nm and MWCNT length of 1.06 µm as shown in histograms of particle size and MWCNT length distribution in Fig. 3c and d. The wellconnected MWCNT conductive networks adhered to the surface nanocluster particles improved the electrochemical recombination effect of the cathodic ions [21].

FTIR Analysis
The formation of LiNi 0.1 Mg 0.1 Co 0.8 O 2 and fullerene-MWCNT hybrid draped LiNi 0.1 Mg 0.1 Co 0.8 O 2 was further confirmed by FT-IR analysis. Figure 4a shows the mode of vibrations of synthesized pristine layered nanostructure of LiNi 0.1 Mg 0.1 Co 0.8 O 2 cathode material in the region between 500 and 1400 cm −1 . The peak at 1139 cm −1 is common for all Co oxide anchored species. The peaks between 800 and 1050 cm −1 are due to the stretching vibrations of terminal M=O and below 800 cm −1 (734 cm −1 ) is due to bridged M-O-M stretching vibration [22][23][24]. In the spectrum (Fig. 4b) of fullerene-MWCNT hybrid draped LiNi 0.1 Mg 0.1 Co 0.8 O 2 , the peaks at 556, 594 and 1120 cm −1 could be related to the F 1u mode of active vibrations of the fullerene regime [25]. The FTIR analysis confirms the presence of fullerene-MWCNT hybrid in the synthesized composite of cathode material.

HRTEM Analysis
The   The HRTEM image of fullerene-MWCNT hybrid draped LiNi 0.1 Mg 0.1 Co 0.8 O 2 nanocomposite shown in Fig. 5h discloses well-defined lattice fringes with a separation of 0.471 nm corresponding to (003) plane of LiNi 0.1 Mg 0.1 Co 0.8 O 2 and another separation of 0.344 nm corresponding to the graphitic crystalline planes of (002) respectively.
Minimizing the particle size to about 12 nm effectively decreases the length of the diffusion path of the charging species and favors high rate capability [27].

Micro-Raman Spectral Analysis
Micro-Raman spectra of pristine LiNi 0.1 Mg 0.1 Co 0.8 O 2 , fullerene-MWCNT hybrid draped LiNi 0.1 Mg 0.1 Co 0.8 O 2 and fullerene-MWCNT hybrid are shown in Fig. 6. According to the theoretical factor-group study, there are two Raman active modes of E g and A 1g for the layered metal oxides with the R3m as a space group [28]. The LiNi 0.1 Mg 0.1 Co 0.8 O 2 Raman spectrum (Fig. 6a) shows bands of E g and A 1g at 540.78 cm −1 and 649.63 cm −1 .
In  Fig. 6a. This is due to the conformational changes that occurred within LiNi 0.1 Mg 0.1 Co 0.8 O 2 during composite formation.
The Raman spectrum of fullerene-MWCNT hybrid as shown in Fig. 6c displayed a strong G band at 1574.33 cm −1 which arises from the tangential C-C bond stretching with sp 2 hybridized carbon atoms in an ordered graphitic structure and corresponds to a splitting of the E 2g stretching mode of crystalline graphite. A weak D band at 1349.41 cm −1 arises due to the disorder-induced in sp 2 -bonded carbon. The intensity of the D-band corresponds to the open-ended sp 3 hybridized carbon atoms, associated with the measurement of disorders in the C-C bonds [29]. A 2D band at 2696.51 cm −1 is the characteristic band of graphene. This observation again confirms the non-covalent interaction of LiNi 0.1 Mg 0.1 Co 0.8 O 2 with fullerene-MWCNT hybrid [29].    Fig. 7f mainly represents graphitic carbon [30]. As reported in the literature [15] the modification of NCM with graphene oxide can lead to the transition metals changing their oxidation states and issue in low discharge capacity and poor cycling performance. In this work, the observed XPS peaks of fullerene-MWCNT hybrid draped LiNi 0.1 Mg 0.1 Co 0.8 O 2 nanocomposite are almost identical with those of LiNi 0.1 Mg 0.1 Co 0.8 O 2 , which indicates that the incorporation of fullerene-MWCNT hybrid did not affect the oxidation state of the elements by which the cathode material maintains its structural stability. This is confirmed by the high discharge capacity and good cycling performance of Li/LiNi 0.1 Mg 0.1 Co 0.8 O 2 + fullerene-MWCNT cell.    Fig. 9b. For the 0.2C rate, the cell delivered a discharge capacity of 181 mAh g −1 with a charging capacity 173 mAh g −1 with 95% of coulombic efficiency. At higher C-rates the discharge capacity reduced to 152 mAh g −1 at 1C and 147 mAh g −1 at 2C. The discharge capacity decreased with an increase in the C-rate.

Cyclic Voltammetry of Li/LiNi
The typical galvanostatic charge/discharge cycling profile of Li/LiNi 0.1 Mg 0.1 Co 0.8 O 2 + fullerene-MWCNT cell at 0.1C rate between 2.5 and 4.5 V is depicted in Fig. 9c. The cell was found to deliver a discharge capacity of 208 mAh g −1 with a charging capacity of 211 mAh g −1 during its first cycle with 99% of Coulombic efficiency, with a decrease of the discharge capacity to 202 mAh g −1 at its 5th cycle with a fade in the capacity of 1. Ma et al. reported that the nanocomposite graphene oxide-coated -SnO 2 -NCM electrode for lithium-ion batteries synthesised by wet chemical method delivered a discharge capacity of 197 mAh g −1 [15]. Tang et al. stated that the high power cathode LiNi 0.5 Mn 1.5 O 4 nanorods wrapped with graphene nanosheets for lithium-ion batteries delivered a discharge capacitiy of 122 mAh g −1 [31]. Li et al. showed that the NCM/CNT hybrid electrode for lithium-ion batteries prepared by wet chemical method delivered a initial discharge capacity of 196 mAh g −1 [5], Tian et al. reported that a unique 3D porous graphene aerogel wrapped LiNi 0.6 Co 0.2 Mn 0.2 O 2 nanoparticle composite (NCM@GA) prepared via a facile coprecipitation delivered a initial discharge capacity of 189.9 mAh g −1 [32].
As a comparision with the other materials mentioned in the literature, the synthesised fullerene-MWCNT hybrid draped LiNi 0.1 Mg 0.1 Co 0.8 O 2 nanocomposite favors an increased discharge capacity, facilitating the transfer of lithium ions across the active material/electrolyte interface, as well as the transfer of electrons from the current collector to the active material. This unique Li/ LiNi 0.1 Mg 0.1 Co 0.8 O 2 + fullerene-MWCNT cell provides a shorter lithium-ion diffusion path and good electronic conductivity, resulting in the demonstration of excellent electrochemical properties.
In addition, the factors that may have contributed to the superior electrochemical performance of the fullerene-MWCNT hybrid draped LiNi 0.1 Mg 0.1 Co 0.8 O 2 nanocomposite electrode are improvement in the structural stability, a decrease in the disorder of metal ions in the lattice, suppression of the dissolution of transition-metal ions, and phase transitions, removal of HF from the electrolyte solution, and a reduced amount of heat production during charge-discharge processes. Also, the improved electrochemical performance of the fullerene-MWCNT hybrid draped LiNi 0.1 Mg 0.1 Co 0.8 O 2 nanocomposite was attributed to an increase in the grain connectivity and high electronic conductivity. The good elasticity between the graphene layers of MWCNT can buffer the volume changes of the oxide materials, inhibit the agglomeration between the oxides and the powdered electrode materials, and obviously improve the cycling performance of the electrode materials. The Warburg factor σ was calculated from the impedance spectra using Eq. (1) in which ω, R s, and R ct represent the angular frequency, solution resistance, and charge transfer resistance respectively [34]. The Lithium diffusion coefficient D was calculated using Eq. (2) in which the parameters R, A, T, F, C, σ and n represent the universal gas constant, the surface area of the as-prepared cathode, absolute temperature, Faraday constant, lithium-ion concentration, Warburg factor, and the number of electrons/ molecule in oxidation process respectively [35]. The Warburg factor was calculated from the graphical plot between Z′ and ω −1/2 as shown in Fig. 10b. Using the calculated value of the Warburg factor σ, lithium diffusion coefficients were determined and tabulated in Table 1. Using Eq. (3) exchange current density I o was calculated [34] (1)

Impedance
The lithium diffusion coefficient increased from 5.13 × 10 -13 to 8.   Facilitated by the interconnecting and conductive fullerene-MWCNT hybrid network, this newly designed nanocomposite material offers a significantly larger initial discharge capacity of 208 mAh g −1 with good cycling stability and stable electrochemical reversibility. As fullerene-MWCNT hybrid improves the particle-particle connectivity, it constructs an efficient Li-ion and electron channel, which significantly enhances the Li-ion diffusion rate and reduces the charge transfer resistance.