molybdenum dioxide nanocomposite for high-performance Lithium-ion batteries

Journal Pre-proof Self-sacrificing Template Based Hollow Carbon Spheres/Molybdenum dioxide Nanocomposite for High-Performance Lithium-Ion Batteries Tahir Rasheed, Faran Nabeel, Ahmad Naveed, Saadat Majeed, Tauqir A. Sherazi

PII:

S2352-4928(19)30483-0

DOI:

https://doi.org/10.1016/j.mtcomm.2019.100694

Reference:

MTCOMM 100694

To appear in:

Materials Today Communications

Received Date:

23 July 2019

Revised Date:

25 September 2019

Accepted Date:

10 October 2019

Please cite this article as: Rasheed T, Nabeel F, Naveed A, Majeed S, Sherazi TA, Self-sacrificing Template Based Hollow Carbon Spheres/Molybdenum dioxide Nanocomposite for High-Performance Lithium-Ion Batteries, Materials Today Communications (2019), doi: https://doi.org/10.1016/j.mtcomm.2019.100694

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Self-sacrificing Template Based Hollow Carbon Spheres/Molybdenum dioxide Nanocomposite for High-Performance Lithium-Ion Batteries

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Tahir Rasheed*1, Faran Nabeel1, Ahmad Naveed1, Saadat Majeed2, Tauqir A. Sherazi*3

School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai

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200240, China; 2 Division of Analytical Chemistry, Institute of Chemical Sciences, Bahauddin Zakariya University, Multan 60800, Pakistan. 3 Department of Chemistry, COMSATS University

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Islamabad, Abbotabad Campus, Abbottabad 22060, Pakistan.*Corresponding authors Email:

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Graphical Abstract

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[email protected] (T. Rasheed) [email protected] (T.A. Sherazi).

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Research Highlights

 Template-Sacrificial synthesis of Hollow polymer spheres  Nanocomposite material with uniform pore size for high performance LIBs.

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 Excellent charge / discharge capacity and columbic efficiency.  High cycling stability

Abstract

Uniform pore sized, hollow carbon spheres encapsulated with molybdenum dioxide (HCSs/MoO2) were prepared via a template-free methodology that utilizes the solution based self-assembly of alternating copolymer followed by the carbonization. The as-prepared HCSs/MoO2 nanocomposite material have a uniform pore size distribution and successfully applied as an electrode material for lithium ion batteries (LIBs). Further, the electrochemical properties of HCSs/MoO2 nanocomposite were also evaluated in this manuscript. The synthesized HCSs/MoO2

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nanocomposite material exhibit excellent discharge capacity of 1395 mA h g-1 for the first cycle

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and 1094 mA h g-1 over the 100 cycles at a current density of 500 mA g-1. Such excellent maintenance of high capacitive value may be attributed to the uniform porosity of the HCSs/MoO2

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composite material. This uniform pore size distribution greatly enhances the transportation of

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lithium ions (Li+) and shielding the volume changes in the composite matrix. A number of advantages including structural controllability, delicate reaction conditions, and great

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electrochemical performance render the HCSs/MoO2 nanocomposite material as a potential anode material for high-performance Li-ion batteries.

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Keywords: Self-assembly; hollow carbon sphere; molybdenum dioxide; lithium-ion battery; anode material;

Introduction

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Lithium-ion batteries (LIBs) are the prospective alternate among energy storage devices. The emerging features of LIBs such as environmental benignity and high energy density have commanding influence in the rechargeable energy storage field since their commercialization [1]. Considering the extensive features and widespread popularity, energy products and portable electronic devices the corresponding stipulation of LIBs is expected to increase steadily in the next

few years. On the bases of excellent chemical and physical properties such as low cost, high conductivity, and stability, graphite has widely been applied as an anode material for the purpose. However, the poor rate competency and short/low theoretical capacity value (372 mA h/g) of the material (graphite) significantly limits its usefulness/effectiveness in commercial storage devices. This necessitates an emergence in the development of a material having a large current discharge

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as well as reversible capacity and can be served as an anode material for LIBs [2,3]. Production of functional nanomaterials on a large scale, especially for self-assembled nano-objects, is still an

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important challenge. Several carbonaceous materials including carbon-based nanofibers, nanotubes (CNTs), reduced graphene oxide, hollow carbon spheres (HCSs), ordered porous

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carbon, and their nano/micro composites are being used as an anode material [4-9]. These materials

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have outstanding electrochemical performances due to their electrochemical kinetics and cycling stability. Hollow carbon spheres (HCSs) encapsulated with transition metal oxide (TMO) are

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highly attractive among all these morphologies. Because of their high performance, relative abundance and high theoretical capacities, the transition metal oxides (NiO, CuO, Fe2O3 and

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Co3O4 etc.) are considered as promising candidates for the anode materials of LIBs [10-20]. However, material pulverization, low intrinsic conductivity, quick fading capacity/ fast capacity decline and severe volume changes (≥ 200%) are the major drawbacks of the TMOs as anode materials in LIBs. Continuous efforts are/have been being put forward to circumvent these issues,

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which are mostly related with mechanical stability and electrical conductivity by fabricating the composite materials [21-25]. These nanocomposite materials have a significant impact on the battery performance by minimizing the stated issues. Among these composite materials, MoO2 encapsulated with alternating copolymer hollow sphere are supposed to be promising materials for the LIBs. The extraordinary properties are based on several nano-specific factors. Primarily, the

intercalation or extraction pathways in LIBs are abridged by the small diameter of nanosized MoO2. This enhances the transportation of Li+ into the interior of MoO2 based alternating copolymer hollow spheres, allowing high charging and discharging rates [26-28]. Secondly, the significantly enhanced surface area of carbon-based hollow sphere active material increases the storage ability afar that of conventional materials [29]. Thirdly, the carbon-based hollow spheres

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act as an efficient conductive, flexible and continuous matrix in the nanocomposite to accommodate the large volume changes of MoO2 NPs and even further keeps fractured MoO2

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pieces trapped within the conductive matrix, leading to enhanced cycle stability [30]. Hitherto, several attempts have been devout to optimize and design the structural features with special

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emphasis on porosity, the concentration of TMO and cavity dimensions of MoO2 encapsulated

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composite materials for LIBs. However, immense space still exist for enhancement in electrochemical performances of LIBs [31,32]. Various synthetic methods such as self-assembling

nanocomposite [33-35].

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spray drying and in situ synthesis are employed for the fabrication of such type of TMO based

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Alternating copolymers are a class of polymeric precursors, which reveal a distinctive structure through alternation of monomeric unit in the polymeric backbone. This unit permits the selfassembling of copolymers into various morphologies with hydrophilic or hydrophobic realms of identical dimension instead of the dispersity of the copolymers [36-41]. Herein, benefiting from

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the peculiar amphiphilic character of alternating copolymers our group reported the template-free methodology for the fabrication of porous HCSs having uniform micropores [42]. Facile thiolepoxy click reaction between 9,9′-bis(4-glycidyloxyphenyl)- fluorine (BGOPF) and 1,4dithiothreitol (DTT) was employed for the synthesis of alternating copolymer (BGOPF-a- DTT). Further, the (BGOPF-a- DTT) was self-assembled into the hollow polymer spheres in a mixed

solvent DMF/water. After obtaining the uniform hollow spheres, the carbonization of the formed aggregates was carried out in a tubular furnace at 800 °C under N2 atmosphere to obtain the hollow carbon spheres (HCSs) of uniform size distribution. Taking the advantage of the uniformity of the microspheres, HCSs were converted into a nanocomposite material by combining with MoO2 followed by carbonization at 600 °C in 95% Ar and 5% H2 atmosphere. The finally obtained

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nanocomposite was designated as (HCSs/MoO2) and exhibit excellent reversible capacity as a potential electrode in LIBs. The as designed nanocomposite electrode reveals the initial discharge

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capacity of 1395 mA h g-1 for the first cycle and maintain a value of 1094 mA h g-1 over the 100 cycles at a current density of 500 mA g-1. Therefore, it can be concluded that the HCSs/MoO2

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nanocomposite electrode has abundant active sites for storage of electrochemical, high rate

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performance and short ion-transportation path. The detailed synthetic route for the formation of

Experimental Section

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Materials and Methods

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HCSs/MoO2 nanocomposite is illustrated in Scheme 1.

The detailed instrumentation, reagents used and preparation and self-assembly of the alternating copolymer has been documented in supporting information.

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Preparation of Hollow carbon spheres The as prepared alternating copolymer based hollow carbon spheres were carbonized at 800 °C in a tubular furnace with a ramp rate of 3 °C/min under nitrogen atmosphere and kept at this temperature for 3 h. After designated time interval, the hollow polymer spheres were converted into hollow carbon spheres (HCSs). After lowering down the temperature of tubular furnace to

room temperature, the samples were taken out and grind to fine powder to get the uniform sized carbon spheres. Preparation of Hollow carbon spheres/MoO2 composite (HCSs/MoO2) In a typical synthesis of HCSs/MoO2 nanocomposite material, the HCSs powder (0.1 g) was dispersed in concentrated nitric acid (HNO3) by sonicating for 30 min. Then the dispersion was

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heated at 70 °C for 1 h to induce the hydrophilicity. The concentrated nitric acid was removed.

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After that, the phosphomolybedic acid (1.58 g) was dispersed in 20 mL of ethanol and added dropwise to the HCSs powder with continues stirring at room temperature. The resulting mixture

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was magnetically stirred overnight at room temperature to impregnate the solution on HCSs. Then, the mixture was heated at 75 °C to remove the solvent. After completely drying the wetted carbon

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material the powder was carbonized at 600 °C at a ramp rate of 2 °C min-1 in 95% Ar and 5% H2

powder.

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Results and discussion

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atmosphere for 5 h. Finally, the obtained HCSs/MoO2 nanocomposite powder was grind to fine

The concept of click chemistry was used in the preparation of alternating copolymer P(BGOPF-aDTT). Briefly, the epoxy groups on 9,9′-bis(4-glycidyloxyphenyl)- fluorine (BGOPF) and thiol

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functionalities of 1,4-dithiothreitol (DTT) were reacted in the presence of 1,5,7triazabicyclo[4.4.0]dec-5- ene (TBD) as a catalyst in ethanol/DMF (1:1 v/v) mixed solvent at room temperature (Scheme 2). The successful synthesis of the alternating copolymer is demonstrated by 1H NMR spectroscopy. After polymerization, the proton signal at 2.14 ppm, which belongs to the thiol group in DTT and the peaks at 2.68, 2.82, 3.27 ppm, which are attributed to the epoxy group in BGOPF disappear. Meanwhile, new peaks at 5.16 and 4.69 ppm attributed to –OH appear

clearly (Figure 1a). Due to the stronger electron-donating effect of the newly formed groups, the proton resonances ascribed to benzene rings shift up-field (from 6.85-7.92 ppm to 6.77-7.87 ppm). The FTIR spectrum is displayed in Figure 1b. A strong broad peak is found at 3384 cm-1 which is attributed to stretching vibration of -OH, indicating that these hydroxyl groups might form strong multiple hydrogen bonds. Due to the presence of benzene ring structures, an absorbent signal is found at 2921 cm-1, which belongs to stretching vibration of aromatic C-H. Meanwhile, the

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skeleton vibrations of aromatic rings are found at 1639, 1495 and 1284 cm-1, respectively. The

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peaks at 822 and 740 cm-1 are attributed to out-of-plain vibration of aromatic C-H and -CH2respectively. The broad peak from 700-500 cm-1 is ascribed to the stretching vibration of C-S. The

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molecular weight and its distribution for P(BGOPF-a-DTT) were studied by GPC measurements.

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As shown in Figure 1c, the number average molecular weight of P(BGOPF-a-DTT) is 9,516 Da and the molecular weight distribution Mw/Mn is 2.76. The thermal properties of P(BGOPF-a-

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DTT) were investigated through TGA and DSC measurements. The decomposition temperature of P(BGF-a-DHBDT) is around 237.6 oC (Figure S1a). The glass transition temperature (Tg) is 133.4 C (Figure S1b) and No crystal melting peaks are found on the DSC curve, indicating that

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P(BGOPF-a-DTT) is an amorphous polymer. The high Tg is favorable for the structure stability during the conversion of HPSs to porous HCSs by the high temperature pyrolysis. The selfassembling behavior P(BGOPF-a-DTT) into hollow carbon spheres was carried through the

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dropwise addition of deionized water (100 mL) at a dropping rate of 10 mL h-1 to a 0.5 mg mL-1 solution of P(BGOPF-a-DTT) in 100 mL of DMF. After designated time interval, the solution was subjected to dialysis in deionized water to remove the organic solvent DMF. A turbidity was observe d during the dialysis which might be due to Tyndall effect. This turbidity consolidate the fact that the P(BGOPF-a-DTT) has been self-assembled into some specific morphology. For the

confirmation of morphology the Scanning electron microscopy was used (SEM). The images exhibit that these aggregates have a spherical structure with narrow size distribution (Figure 2a). For the average diameter measurements, the statistical analyses of 200 particles were performed. The average diameter was found to be around 398 ± 76 nm via (Figures 2a). Furthermore, the high-resolution SEM divulge the existence of small holes in the structure of some spherical

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aggregates, designating that these aggregates possibly have hollow structures (Figure 2b). These results were further supported by transmission electron microscopy (TEM). The pictographs

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indicates that the ultrathin slices of the masses embedded in the epoxy resin validating the presence of cavities inside the spheres (Figure 2c and 2d). Both, results consolidated the hollow spherical

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structure of the aggregates. Further, the hydrodynamic diameter of the formed aggregates was

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confirmed by Dynamic light scattering (DLS) technique. The results shows a Dh of 458.7 nm with a polydispersity index (PDI) of 0.263 for the resulting aggregates (Figure S2). The self-assembling

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behavior of the alternating copolymers have further been supported with the help of literature. Previously it has been reported that the alternating copolymer can self-assembled into various

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morphologies such as, nanospheres, sea urchin-like self-assemblies, vesicles and nanotubes etc. (Chen et al. 2015; Xu et al. 2018; Zhang et al. 2018; Rasheed et al. 2019; Li et al. 2017; Li et al. 2019). To the best of our understanding, the aggregation of alternating copolymers into hollow spheres have very few examples. Our group (Li et al. 2019) has already reported the detailed self-

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assembling mechanism. To have a better insight into the practical application of the formed hollow polymeric spheres (HSPs). The as prepared HPSs were thermally treated in a tubular furnace at a temperature of 800 oC in the presence of nitrogen. Initially, the irregular shaped bulk carbon material was obtained as a result of carbonization of HPSs. Therefore, to have a regular morphology and increase the thermal stability, the HPSs were cross-linked with anhydrous

Aluminum chloride (AlCl3) in chloroform (Scholl reaction). These cross-linked polymeric spheres were further subjected to the pyrolysis at 800 °C under N2 atmosphere for 3 h. Moreover, the retention in the morphology of these aggregates was supported by SEM and TEM pictographs. Both, the analyses reveals that the spheres have retained their shape with narrow distribution of size after pyrolysis (Li et al. 2019). Further, the pore size distribution of the hollow carbon spheres

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(HCSs) was carried out by N2 adsorption−desorption isotherm. A typical type IV shape was shown in figure 3a according to IUPAC classification. This curve reveal that the hollow carbon spheres

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have mesoporous pore size and it displays H1 hysteresis loop which is widen at relatively high relative pressure (P/P0 > 0.5) indicating that the excessive mesopores and typical characteristic of

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the spherical structures [43]. The size distribution curve was plotted by using nonlocal DFT

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method (figure 3b). This plotation reveals that the HCSs have uniform mesoporous size distribution. In addition, it can be seen from the plot that there are few micropores in the porous

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structure of HCSs. This hierarchical porous structure with suitable distribution of pore size is the striking feature influencing the performance of Lithium-ion storage during electrochemical

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processes. The micropores are responsible for the storage of Li+ ions where mesopores facilitate the transmission passages to Li+ with a smaller transference resistance. The hierarchical structural and textural investigation of HCSs/MoO2 were further investigated by TEM and HRTEM measurements (figure 4a-c). The figure 4a reveals that the HCSs/MoO2 nanocomposite are

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spherical in shape and contain abundant pores. A rough surface and porous architecture was revealed by figure 4b. Furthermore, Figure 4c shows HRTEM pictograph of HCSs/MoO2 nanocomposite. There appears two fringe spacing represented by arrows in red color in the figure. These fringes are correspond to monoclinic lattice planes of MoO2 respectively. Additionally, to

consolidate the homogeneous dispersion of Mo, C and O throughout the matrix, the EDX analysis was carried out. The Figure 4d-f, presents that all the elements are distributed homogeneously. The X-ray photoelectron spectroscopy (XPS) was carried out to find out the chemical composition of HCSs/MoO2 nanocomposite material. Figure5a portrays the merged spectra of all the component present in HCSs/MoO2 nanocomposite material. There appear four distinctive peaks

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positioned at 233.31, 283, 397.31-414.31 and 530.4 eV and are ascribed to the Mo 3d, C 1s, Mo 3p3/2, Mo 3p ½ and O 1s respectively [44,45]. Further, the detailed spectrum of the Mo 3d has been

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presented in the figure 5b. This high-resolution spectrum shows three doublets of the Mo 3d region.

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One doublet corresponds to 229.5 and 231.9 eV represent the Mo 3d5/2 and3 d3/2. The difference of binding energy appears to be 2.4 eV that is the character of Mo4+ oxidation state of Molybdenum

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(Sun et al. 2011). The peak around 233 eV reveals the presence of small amount of Mo2-C (Gao et al. 2014). Further, the band around 236 eV features that there is Mo(VI) in 3d3/2 form. This

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indicates that the oxidation of Mo may happen during the measurement process [46]. The high resolution XPS analysis of carbon is presented in figure 5c. It can be clearly seen that the

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appearance of signals at 284.8 and 286.6 eV is correspond to the carbon peak as C-C and C-O. Additionally, the peaks at 289.5 and 285.5 represents the O-C=O and Mo bonded carbon respectively. Finally, the O 1s spectrum reveals two peaks positioned at 530.4 and 531.9 eV

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respectively and are designated as the peaks of Mo-O and C-O (Figure 5d) [44,47]. Electrochemical Performance The electrochemical performance of the prepared MoO2/HCSs composite was also investigated in an active material/Li half-cell arrangement. The figure shows the first three cyclic voltammetry curves (CV), displaying an excellent performance. These tests were performed in a potential window from 0.01-3.0 V at a scan rate of 0.1 mV/s. it can be seen from the voltammograms that

first curve is slightly different from the other successive curves. This curve exhibit a marked reduction peak of Li+ insertion around 1.44 V. This may be attributed to the phase transition from orthorhombic to monoclinic. This phase transition owes from the insertion reaction of Li+ in the partially lithiated LixMoO2 during lithium insertion and extraction [45]. On the other hand, a significantly sharp reduction peak at 0.83 V appears in the cathodic scan, indicating the

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oxidation/reduction reaction. It was worth noting that the cathodic peak appearing below 0.3 V in the first discharge process and disappearing in the following cycles was attributed to the

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irreversible reduction of the electrolyte and the formation of SEI layer. Finally, the peak disappears in the next cycles [46,47]. Furthermore, the succeeding curves almost overlap completely,

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revealing high stability and reversibility of the synthesized MoO2/HCSs composite for the lithium

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extraction and insertion reaction (Figure 6a). To further, investigate the electrochemical performance the charge-discharge cycling performance (potential profile) of MoO2/HCSs

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nanocomposite as an electrode material was performed in a voltage range of 0.01-3.0 V and a current density of 500 mA g-1. The figure 6b portrays the charge-discharge performance of 1st,

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10th, 25th, 50th and 100th cycles. The curves present high initial values of charge and discharge capacities in the range of 1085 mA h g-1 and 1395 mA h g-1 respectively, with a columbic efficiency of 77.7 %. This relative great loss in efficiency (22.3 %) may originate due to irreversible reduction of molybdenum oxide (MoO2) to molybdenum (Mo) and some other probable processes such as,

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SEI film formation, decomposition of electrolyte and trapping of lithium in the molybdenum oxide matrix [47]. This is a common phenomenon of most of the anode materials. No significant fading of capacity was noticed in the proceeding cycles (10th, 25th, 50th and 100th). Also, the columbic efficiency increases from 96 % to 99 % and becomes constant. The presence of the charge discharge plateau at 0.6 V and 2.1 V indicates the formation of LixMoO2 through Li+ ion insertion

and extraction process. Furthermore, the cycling performance of the MoO2/HCSs composite material was carried out at a potential window of 0.01-3.0 v and a current density of 500 mA g-1 and shown in figure 6c. The initial charge and discharge capacities were found to be 1021 mA h g-1 and 1430 mA h g-1 respectively, with a columbic efficiency of 71.39 %. This efficiency was increased and stabilized to 99 % in the proceeding cycles accompanied by the stabilization of

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charge and discharge capacities [48,49]. This delayed stabilization is because of the activation process of the electrode and commonly observed in the transition metal oxide based composite

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electrode material. From 20 to 50 cycles a slight descending behavior of the capacity was noticed which ascend from 50 to 60 cycles it reveal an ascending behavior and finally becomes steady

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afterwords. This indicates no pulverization and reactivated process are happening on the electrode

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material during the cycling [50]. Figure 6d portrayed the rate proficiency of the MoO2/HCSs composite electrode material as the current density was gradually amplified from 20 to 500 mA gand finally returned back to the initial current density of 20 mA g-1. A reversible capacity of 1300

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mA h g-1 was delivered by MoO2/HCSs composite electrode at 20 mA g-1. Similarly, at higher

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current densities i.e.50, 100, 200, and 400 mA g-1 the stable values of capacities of 1062 mA h g, 981.6 mA h g-1, 882 mA h g-1 and 774 mA h g-1 were revealed by the MoO2/HCSs composite

electrode respectively. Further, when the electrode was cycled at even higher value of current such as, 500 mA g-1 the average value of the reversible capacity was not less than 610.7 mA h g-1. This

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value is still greater than the theoretically calculated capacity of graphite (372 mA h g-1). It was interesting to note that when the current density was decreased to 20 mA g-1 again, the value of the reversible capacity was raised to 988 mA h g-1 quickly. This increase/retention of capacity clearly indicate that the MoO2/HCSs composite electrode can work with great stability and efficiency even at extremely high current density.

Furthermore, the electrochemical impedance spectra (EIS) of HCSs/MoO2 nanocomposite were also carried out as fresh cell, after 1st, and 10th charge-discharge cycles in a frequency range of 0.1 Hz to 100 kHz. As shown in figure 7, the Nyquist plots of fresh and cycled cell shows semicircles in the high frequency range while an inclined line is shown in low frequency range. The fresh cell in high frequency zone shows a depressed semicircle as a result of charge transfer

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resistance in high frequency region. It can also be seen that the size of the semicircle decreases in the curves after 1st and 10th cycles indicating that the nanocomposite HCSs/MoO2 offering less

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contact and charge resistance. This low resistance attributed to the high content ratio of carbon.

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Conclusion

Conclusively, the alternating copolymer hollow carbon spheres were synthesized through

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template free approach and were converted into HCSs/MoO2 nanocomposite via encapsulation of

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MoO2 accompanied by successive carbonization at 800 0C and 600 0C under N2 and Ar atmosphere. The structural features of the formed nanocomposite such as hollow architecture, high

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porosity and large surface area, endow the HCSsMoO2 nanocomposite with extensively greater Li storage performance. Encouragingly, reversible and high capacity accompanied by excellent stability was delivered by as designed HCSs/MoO2 nanocomposite as an electrode for LIBs. The high capacitive value of 1395 mA h g-1 for the first cycle and 1094 mA h g-1 over the 100 cycles

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at a current density of 500 mA g-1. The special structural features of HCSs/MoO2 efficiently abridges the electron diffusion and Li+ ions paths, and inhibits the agglomeration of MoO2/HCSs/MoO2. The structural maintenance guarantees enhanced electrical conductivity, and primes the enhanced lithium storage features as discussed above. More importantly, the structural features of the formed HCSs/MoO2 nanocomposite material can possibly be applied to various

types of metal oxides encapsulated composite material as high-capacity electrodes for LIBs and other storage devices. .

Conflict of Interest The authors declare no competing interest in any capacity.

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Acknowledgment The authors gratefully acknowledge the financial support of Pakistan Science Foundation

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(PSF/NSFC-Eng/KP-COMSATS-ABT-04) and Higher Education Commission of Pakistan (20-

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3684/R&D/HEC/14)

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Scheme 1. Schematic illustration of the synthetic methodology of porous HCSs and HCSs/ MoO2 nanocomposite with uniform micropores through the self-assembly of P(BGOPF-a-DTT). Scheme 2. Synthetic route of P(BGOPF-a-DTT). Figure 1. Characterizations of the P(BGOPF-a-DTT). (a) 1H NMR spectrum in DMSO-d6, (b) FTIR spectrum, (c) GPC curve.

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Figure 2. Characterizations of HPSs. SEM image (a) Low-magnification (b) high-magnification

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(c, d) TEM images at Low and high magnification.

Figure 3. (a) N2 adsorption-desorption isotherms and (b) pore size distribution of HCSs/MoO2

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nanocomposite.

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Figure 4. (a, b) TEM, images, (c) HR-TEM image of HCSs/MoO2 nanocomposite, (e) EDX

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mapping results for HCSs/MoO2 nanocomposite.

Figure 5. (a) XPS spectrum of HCSs/MoO2 nanocomposite (b) survey spectrum; high-resolution

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Mo 3d, (c) C 1s, and (d) O 1s.

Figure 6. (a) Cyclic voltamograms at a scan rate of 0.1 mV/s in a voltage range of 0.01-3.0 V vs. Li/Li+ (b) Galvanostatic discharge/charge curves at a current density of 500 mA g-1 (c) cycling performance at 500 mA g-1 for 100 cycles with the corresponding coulombic efficiency (d) rate

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performance at various current densities of 20 mA g-1, 50 mA g-1, 100 mA g-1 and 200 mA g-1, 400 mA g-1 and 500 mA g1 of HCSs/MoO2 nanocomposite electrode. Figure 7. Electrochemical impedance spectra of the HCSs/MoO2 nanocomposite before cycling in a frequency range of 0. 1 Hz to 100 kHz.

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