Graphene-Based Coronal Hybrids for Enhanced Energy Storage

Functional materials with designer morphologies are anticipated to be the next generation materials for energy storage applications. In this manuscript, we have developed a holistic approach to enhance the surface area and hence the properties of nanostructures by synthesizing coronal nanohybrids of graphene. These nanohybrids provide distinctive advantages in terms of performance and stability over vertically stacked nanocomposites reported in literature. Various double hydroxide materials self-assembled as coronal lamellae on graphene shells have been synthesized and systematically studied. These coronal nanohybrids result in about a threefold increase in energy storage capacity as compared to their traditionally synthesized nanocomposite counterparts. The 3D graphene-based nanofibrils in the synthesized coronal nanohybrids provide mechanical support and connect the nodes of the double hydroxide lattices to inhibit restacking. Complex morphologies such as coronal nanostructures increase the interaction surface of the nanostructure significantly. Such an approach is also expected to bring a paradigm shift in development of functional materials for various applications such as sensors, energy storage, and catalysis.


Introduction
Engineering morphologies of nanostructures by combining diverse configurations to enhance material properties is an effective approach to synthesize advanced functional materials. The design of novel and sophisticated architectures however requires various innovative approaches. Such designer morphologies provide distinctive advantages in terms of performance and stability over traditionally synthesized nano-composites. Impending innovations in 2D materials are expected to involve expansion of compositional chemistry of interfacial layers [1][2][3]. Incidentally, incorporation of low-dimensional materials with carbon based nanostructures is an effective approach for synthesizing materials and offers combined advantages of both [4][5][6]. Amongst various low-dimensional materials, 2D materials such as layered double hydroxides (LDHs) have demonstrated great potential to form nanocomposites with many functional nanomaterials [7][8][9][10][11][12]. These are inorganic clays of layered materials with positively charged brucite-like layers and posses several interesting features such as tuneability in composition, structure, morphology. However, there are several challenges associated with the stability and energy storage in nanostructures using LDHs. They aggregate easily [13,14] resulting in compromised the energy storage capacity, lack of conductivity and instability [15]. This leads to parasitic reactions resulting in constrained electrochemical performance. Thus, a strategic approach needs to be developed to design and synthesize smart material architectures. Recent progress in the synthesis of graphene [16,17] based hybrid materials have resulted in a plethora of composites. The major bottleneck is the restacking of the composites with no persistent distinctive nanostructured features [18,19]. These issues can be mitigated by controlled synthesis of core shell materials to develop distinctive and non-traditional graphene frameworks similar to holey graphene [20] or crumpled graphene nanonetworks [21]. Thus one can exploit these designer nano hybrids to their full potential [22].
In this perspective, we have developed a holistic approach in designing and synthesizing 3D coronal architectures of hybrid materials with porous graphene (PG) based nanowebs. These obliterate the boundaries between inorganic LDH materials that exhibit bulk redox reactions [23][24][25] and carbon materials that accumulate charge owing to the surface limited processes [26][27][28]. These architectures are synthesized with controlled geometry by encapsulating coronal hybrids of ultrathin LDH nanosheets of high redox activity, self-assembled radially over functionalized graphene shells, with complementary functionalities. The structural features of coronal hybrids with graphene-based nanostructures introduce reversible wettability and modulation of the chemical potentials with improved rate capability. Additionally, the strain produced in the coronal hybrids due to induced cycling procedures can be relaxed due to the corrugations present on the graphene-based nanonetworks. High resolution STEM and TEM micrographs show evidence of coronal structural features. Comparative electrochemical studies suggests that synthesized coronal hybrids demonstrate good electrochemical stability, rate capability, better electrochemical performance along with enhanced threefold increase in charge storage as compared to their traditionally synthesized counterparts. X-Ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS) analysis and elementally mapped micrographs suggests fine control over the hierarchical structure of the LDH materials. These structures are highly desirable owing to their ability to control the ionic interactions in an electrode-electrolyte interface. Such interactions have been tailored through porous 3D graphene like nanowebs (3D-PG) which not only prevents restacking but also introduce redox-active organic groups that can electronically communicate with the LDHs. Thus, these nanoarchitectures are expected to create a paradigm shift in development of materials not only for energy storage applications but also others, where surface interactions are extremely crucial, such as catalysis [29,30], sensors [31,32], drug delivery [33,34] and flame retardants [35,36] etc. 5

LDH@PG
Graphene oxide (GO) shells are synthesized using template assisted synthesis route, prior to the self-assembly process of coronal lamellae to for coronal hybrids. GO solution (detailed procedure for GO synthesis is provided in supplementary information as MT-1) is taken and refluxed in a beaker. A mixture of porous silica spheres (0.8 g) (synthesis of porous SiO 2 spheres is detailed in supplementary information as MT-2) and DMF solution (50 mL) is sonicated in a round bottom flask for about 1 hour using an ultra-probe sonicator.
Subsequently, (3-Aminopropyl) triethoxysilane is added to this flask. The flask is placed in an oil bath and the temperature of the solution is raised to 110-120 ˚C. GO solution (1mg mL -1 ) along with Dicyclohexylcarbodiimide (2.5g) is added to this silica suspension. The reaction mixture then is continuously stirred for 24 hrs to obtain core shells of GO coated over silica nanospheres.
To obtain the coronal nanohybrids of LDHs, the GO coated silica core shell templates were dispersed in 50 mL alcoholic emulsion and sonicated for about 30 minutes. Hexamine (0.56 g) is added to the alcoholic emulsion and sonicated for another 30 minutes. The metallic nitrate precursors (Nickel nitrate hexahydrate, Cobalt nitrate hexahydrate or Manganese nitrate tetrahydrate) are added for synthesis of their respective LDHs. These are taken with a trivalent metal ratio maintained closely around 0.2-0.3. Later, the obtained solution is sealed in an autoclave reactor vessel with reaction temperature maintained at 90˚C for 12 hours. The final product is obtained after washing the samples with water and ethanol several times using centrifugation. In order to perform comparative study, the pristine LDH samples were also prepared under same conditions of temperature and concentration.

Characterizations
The surface morphology of the synthesized nanostructures was imaged using a field-emission scanning electron microscope (FEG-SEM, JEOL JSM-7600F FEG-SEM) and high-resolution transmission electron microscope (HR-TEM) equipped with an energy dispersive X-ray spectroscopy (EDS) (Thermo Scientific, Themis 300 G3) detector. The crystal structure of the samples was investigated using powdered X-ray diffraction (XRD) technique in the 2θ range of 5-80° at a scan speed of 4 min -1 (Panalytical X'Pert Pro with Cu K irradiation at a wavelength of 0.1542 nm). The surface chemical composition and valance states were examined using X-ray photoelectron spectroscopy (XPS) (AXIS SUPRA) with an Al Kα radiation. To identify the functional groups, FTIR spectroscopy was performed using 3000 Hyperion Microscope with Vertex 80 FTIR System (Bruker). The vibrational modes of the samples were investigated using Raman spectroscopy (HR-800-UV confocal micro-Raman spectrometer). The N 2 adsorption-desorption isotherms and pore size distribution of the samples were measured using a Brunauer-Emmett-Teller (BET) surface analyser (Quantachrome, Autosorb).
The electrochemical studies were performed using Bio-Logic SP-300 Potentio-Galvanostat. The electrochemical evaluations were carried out using cyclic voltammetry, galvanostatic charge-discharge studies and impedance spectroscopy using a three electrode configuration in 3 M aqueous KOH solution as an electrolyte. A platinum mesh with a cross sectional area of 1*1 cm 2 and thickness of 0.1 mm was used as the counter electrode during the measurements. Ag/AgCl was used as reference electrode for the measurements. The working electrodes were fabricated using the active materials, conductive carbon black and polyvinylidene fluoride (binding agent) in a weight percentage of 75:20:5 respectively. The slurry made using these ingredients was pasted on to a Ni substrate and dried overnight in vacuum chamber. The specific capacities were calculated from galvanostatic charge 7 discharge profiles using the equation, C s = Q / m. Here C s is specific capacity (Cg⁻ 1 ), (Q= I d t d) , t d is the discharge period (s), I d is the constant discharge current (A) and m is the active mass loading of each electrode.  The synthesis of coronal nanohybrids wrapped in intricate nanowebs requires a soft template-based approach. A schematic showing detailed synthesis protocol of these nanostructures is shown in Figure 1. In order to achieve this, silica nanoparticles were injected into a solvent medium under agitation with oxidized graphene. This resulted in interfacial growth of GO shells on the silica spheres due to acylation mechanism and formed uniform dispersion. These were then separated by centrifugation. Subsequently LDH nanosheets self-assemble during hydrothermal reaction over the SiO 2 /GO core shell structures and overtime coronal hybrid assemblies along with 3D fibrillar graphene like nanowebs were produced. The SiO 2 etches out self sacrificially resulting in formation of reduced graphene oxide (rGO) shells. This method has been used to demonstrate synthesis of three representative configurations of transition metal coronal hybrid structures i.e. Co-Mn LDH, Ni-Mn LDH and Ni-Co LDH self-assembled over GO shells embedded in a PG network.

Bio-mimetic 3D-spherical coronal hybrids and characterizations
Carefully designed porous silica core assists in the structure-building and are eventually  in table 1 below. Table 1. Calculated lattice parameters and crystallite sizes for the pristine LDH and hybrid samples The homogenously layered cationic distribution in the synthesized nanostructures is assisted by electrostatic interaction of the cationic layers [3,38]. It is understood to be a major factor that favours the crystallization of LDHs. The lattice parameters depend on various factors such as size and amount of interlayer anions, hydration, cation−cation separation [39,40]. Further, the presence of possible heterogeneity in finely dispersed crystalline LDHs with some order-disorder in cationic distributions cannot be ignored [41,42]. The inter-planar spacing is finely tuned with the different compositions. The type of anions intercalated between the interlayers can be controlled by restricting the type and quantity of reagents. The

Sample
Lattice  The small LDH crystallites assemble uniformly over the 3D-PG skeletal structure that enables access to all active intercalation sites, leading to high specific capacities and fast ion diffusion [43]. However, due to extreme proximity of the LDH nanosheets, the 3D-PG signal was too weak to be observed in XRD.
The surface morphology of representative spherical coronal nanohybrid assemblies of Ni-Mn, Co-Mn and Ni-Co LDH shown in Figure 2 Eventually, these nanowebs enhance the ionic current and electronic current conduction rate.
In addition to these images, respective FEG-SEM and HR-STEM micrographs for pristine showing the prominence of the architectural composition in size reduction of sp 2 in-plane domains. As a consequence, there is also a disorientation in the crystal structure of 3D-PG through cross-linking of LDH lamellae [45]. The shifts observed in the Raman spectra suggests that more localized sp 3 defects are introduced into the sp 2 carbon structure [46].  with the oxygen sites or π bands of the rGO core shells, establishing a strong resilience with hydrogen bond [49]. Evidently, there are also weak van der Waals interactions between the LDHs and rGO sheets. Thus, the strong interactions between the LDHs and 3D-PG protect the 3D architecture. Therefore, even high values of current densities and repeated cycling was unable to break the interactions between the LDHs and 3D-PG structure with charge storage processes happening due to redox mechanisms in LDHs and charge-adsorption/desorption in 3D-PG.
To have a better understanding of the synthesis process of 3D-PG, HR-TEM micrographs were recorded at various stages of synthesis and are shown in Figure 4. Figure   4(a) shows GO encapsulation over SiO 2 templates, while Figure 4   The surface areas of pristine Ni-Co LDH and Ni-Co LDH@3D-PG was calculated to be 20.6 m 2 g −1 and 195.4 m 2 g −1 respectively. Further the hysteresis loop for Ni-Co LDH@3D-PG coronal hybrid is of type H3 which is a characteristics of a mesoporous structure. In addition, the pore size distribution is provided in Figure 4(f), which shows large volume of pores in case of the coronal hybrids. The BET measurements suggests that interfacial contact is established between the LDH lamellae and the 3D graphene structures enabling efficient charge transfer mechanism by reducing the ion diffusion length.

Charge storage mechanism in the coronal hybrid architecture
The 3D-PG nano-networks eases the electrolytic access and enhances the overall specific charge storage capacity compared to the solid matrix or aggregated sheets of the active material as shown in Figure 5 It assists in dispersion of the LDH nano sheets to avoid self-aggregation. This helps in creating extra active sites which help in improving the electrochemical performance. Hence, the specific capacity of the hybrid materials is observed approximately three times higher than the pristine LDH materials as seen in Figure 5(c). In addition to offering high specific capacity, the perforations in the coronal structures as shown in Figure 4

Electrochemical evaluation of the hybrid materials
Multi-layered architecture of coronal hybrids significantly enhances the specific surface area along with the porosity. These provide abundant active sites for the migration of ionic species and stimulates the interaction between the electrolyte and active material [51]. Consequently, the coronal hybrids demonstrate higher specific capacity and rate performance compared to instance, coronal hybrid of Co-Mn LDH and Ni-Mn LDH delivered a high specific capacity due to an optimum replacement of Co or Ni by Mn. It is also understood that the structural mitigation by tailoring coronal hybrids using 3D graphene based nanowebs can help in generating a good concentration gradient. However, the Co-Mn LDH@3D-PG delivers a low specific capacity value than Ni-Mn LDH@3D-PG which may be due to highly active Ni 2+ .
During the in-situ growth process, GO shells oxidizes Co 2+ and Mn 2+ to trivalent metallic adsorption/ desorption of ions over the 3D-PG frameworks, which could be assigned as a surface-limited process [53]. The redox peak shifts (anodic peak to a higher potential and cathodic peak shift to a lower potential) of LDH@3D-PGs indicates the effect of polarization, which can be attributed due to the phase separation between pristine LDHs and the presence of 3D graphene framework in LDH@3D-PGs. However, it is anticipated that the presence of carbon framework decreases the polarization by improving the electronic conductivity. The other reason behind the peak separation is due to the overall concentration gradient of Ni or Co affected by the presence of GO during the 3D-PG formation [54]. highly reversible with charge-discharge coulombic efficiency to be more than ~99 %.
Symmetric triangular profile with well-defined plateaus suggests good capacitive behaviour.
One portion of the profile shows a linear discharge profile, indicating a surface-limited charge storage process. The broad extended plateau region represents a major portion of charge storage during discharge due to the redox active species with a low polarization.   with a gradual decrease in the specific capacities is due to fast acting Faradaic reactions that drive incomplete utilization of electrode material. Hence, during the fast intercalation/deintercalation process in LDHs, at higher charge-discharge rates the H + ionic transport is limited to the solid interface and the interaction or the reactivity of H + with the hydroxyl ions is also limited. The coronal hybrid electrodes exhibit a maximum specific capacity at 1 Ag -1 and still retains high specific capacity of 300 Cg -1 , 505 Cg -1 and 755 Cg -1 at 5 Ag -1 for Co-Mn LDH@3D-PG, Ni-Mn LDH@3D-PG and Ni-Co LDH@3D-PG respectively as illustrated in interfacial chemistry of LDHs with graphene based nanowebs. As a consequence, the enhanced battery-type property with a notable specific capacity is still persistent even at high current densities.

Conclusion
To sum up, graphene based designer nanohybrids in form of coronal morphologies have been synthesised with Co-Mn, Ni-Mn and Ni-Co LDH lamellae self-assembled radially outwards as corona on the surface of rGO shells connected by porous graphene networks. The welltailored nano-design, with tuneable interlayer spacing along with modification of the interfacial chemistry with interconnected 3D-graphene like nano-web structures supplement better electronic and ionic conductivities. This in turn leads to low internal resistance that enables in fast charge transfer process with enhanced electrochemical stability. The 3D-PGs are mechanically robust structures and enable the electrodes in tolerating high charging currents. This results in higher energy and power densities in coronal hybrids as compared to pristine and stacked composites reported in literature. The efficient and strategic approach to develop coronal hybrids takes into account the tuneable compositional and structural features of the transition metal layered double hydroxide materials (Ni, Co or Mn) with wellconnected 3D-PG nanoweb architectures that have yielded a high specific capacity. The high rate capability is understood to be due to highly percolated 3D-PGs inside the coronal sphere with radially aligned ultrathin LDH nanostructures. This newly developed nanohybrids can deliver high specific capacities of 776Cg -1 , 984Cg -1 and 1056Cg -1 for Co-Mn LDH@3DPG, Ni-Mn LDH@3DPG and Ni-Co LDH@3DPG respectively with outstanding rate capability.
The holistic design approach in tailoring the morphologies of energy storage materials can lead to the rational design and development of a wide range of functional materials for higher energy and power densities for energy storage applications.

Conflicts of interest
The authors declare no conflict of financial interests or personal relationships that could give the impression to influence this work.