NANOscientific

Jinhua Sun¹, Matthew Sadd², Philip Edenborg³, Henrik Grönbeck³, Peter H. Thiesen⁴, Zhenyuan Xia¹, Vanesa Quintano⁵, Ren Qiu⁶, Aleksandar Matic² ,

Vincenzo Palermo^1,5*

¹Materials and Manufacture, Chalmers University of Technology, Göteborg, Sweden (CU of T). ²Materials Physics, Dep. of Physics, (CU of T). ³Dep. of Physics, CU of T. ⁴Accurion GmbH, Göttingen, Germany. ⁵Institute of Organic Synthesis and Photoreactivity, National Research Council of Italy, Bologna, Italy. 6Microstructure Physics, CU of T. *vincenzo.palermo@isof.cnr.it

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http://advances.sciencemag.org/cgi/content/full/7/22/eabf0812/DC1

This study introduces "Janus" graphene, a novel material with onesided functionalized graphene sheets, enabling reversible sodium (Na⁺ ) intercalation, a key for sodium-ion batteries. Using operando Raman spectroelectrochemistry and imaging ellipsometry, the research demonstrates Janus graphene's effective Na⁺ storage, comparable to graphite in lithium-ion batteries. The material's design, featuring uniform pores and controlled functionalization, along with theoretical insights into its Na⁺ -NH₂ interaction, highlights its potential in advancing sodium-ion battery technology.

INTRODUCTION

Lithium ions exhibit high loading and specific capacity in graphite, forming binary graphite intercalation compounds essential for current batteries [1–3]. In contrast, sodium ions achieve lower loadings and capacity in graphite, with limitations partly attributed to graphite's small interlayer distance [4–8]. Despite sodium's potential as a cost-effective and abundant alternative to lithium, its intercalation is hindered, unlike larger potassium ions or solvated sodium ions, which can intercalate, albeit with drawbacks like significant volume changes and reduced capacity [9–14]. Developing sodium-ion batteries (SIBs) necessitates bare sodium ion intercalation, akin to lithium in existing batteries. Sodium's intercalation challenges relate to its lower ionization potential and larger ionic radius compared to other alkali metals, affecting its electrostatic interaction with graphene [15–18]. We introduce a novel "Janus" graphene, a nanostructured material with one-sided functionalization allowing controlled sodium intercalation, monitored via operando Raman spectroelectrochemistry and imaging ellipsometry, and supported by density functional theory (DFT) calculations (Fig. 1A).

RESULTS

Synthesis of Janus Graphene. We synthesized Janus graphene from high-quality graphene monolayers using chemical vapor deposition (CVD), followed by functionalization with 4-nitrobenzene diazonium tetrafluoroborate (4-NBD) (Fig. 1A). This process, involving aryl radical formation, allows controlled grafting of molecules to graphene, with the ability to adjust the grafting density by altering reaction conditions. Post-grafting, the nitrobenzene groups were converted to aminobenzene groups, creating two graphene variants with different surface chemistries but identical functionalization densities. Characterization techniques like Raman spectroscopy, cyclic voltammetry, and X-ray photoelectron spectroscopy confirmed the grafting and subsequent chemical conversion (Fig. 1, C to E). Notably, the introduction of functional groups increased hydrophilicity, as shown by reduced water contact angles for the modified graphene. Raman analysis revealed the formation of sp3 hybridization and covalent bonding, with an observable shift in the 2D band related to the interaction and separation of graphene layers influenced by functionalization density. The optimal Na⁺ storage was achieved with an intermediate functionalization level, minimizing self-polymerization and ensuring efficient sodium storage capabilities.

Preparation of Stacked Janus Graphene Films Via Supramolecular Interactions. After synthesizing Janus graphene, we developed a method to stack layers without the typical PMMA residue issues, enhancing ion intercalation. Our approach involved using a PMMA graphene bilayer to collect floating graphene layers post-copper substrate etching (Fig. 1A), enabling the construction of a clean, multi-layered structure (Fig. 2A). Raman mapping distinguished the overlapping layers, with an IG/I2D ratio indicative of the reduced interaction between layers due to AB molecules acting as spacers, confirmed by AFM and TEM (Figs. 2B-D). The measured stacking distances, larger than in pristine graphene, suggest AB molecules align perpendicularly between layers, a structure conducive to Na⁺ ion intercalation, as explored through computational methods.

DFT Analysis of Na⁺ Intercalation in Janus Graphene. DFT calculations were employed to explore the optimal arrangement of AB and NB molecules within graphene layers and to assess the energetics of Na⁺ and Li⁺ intercalation. The calculations supported a vertical orientation of molecules acting as spacers, aligning with AFM data. Na⁺ was found to intercalate most effectively when coordinated with the -NH₂ group of AB and positioned in a graphene hollow site, enhancing stability through a synergistic effect. Charge density analysis indicated significant charge transfer, facilitating Na⁺ interaction with graphene and AB. Various configurations were analyzed, demonstrating the highest stability when Na⁺ is near the -NH₂ group, with energy barriers low enough to allow Na⁺ diffusion within the material. This suggests efficient Na⁺ storage in AB-functionalized graphene, with potential for high Na⁺ coverage while maintaining energy stability.

Operando Raman Spectroelectrochemistry for Na⁺ Intercalation in Janus Graphene. Utilizing the uniformity of Janus graphene, we monitored Na⁺ and Li⁺ intercalation/ deintercalation via operando Raman spectroscopy, cyclic voltammetry (CV), and imaging ellipsometry (IES). Initial tests on standard materials confirmed Li⁺ intercalation through a G band shift, while Na⁺ showed no shift in graphite or pristine graphene but did in AB graphene stacks (Fig. 3A, 3B). The G band shift in AB graphene indicated direct Na⁺ intercalation, with optimal intercalation observed at an ID/IG ratio between 0.5 and 0.8, balancing electrical conductivity and active site availability. Unlike Li⁺ in graphite, Na⁺ intercalation in AB graphene was direct, without stages, suggesting uniform intercalation across all layers. Initial cycles showed a low coulombic efficiency due to solid electrolyte interphase (SEI) formation, stabilizing after the first cycle (Fig. 4C). Operando Raman revealed changes in band intensity correlating with Na⁺ intercalation levels, which were reversible and indicative of stable Na⁺ storage. XPS post-discharge confirmed Na+ intercalation, with a high estimated storage capacity significantly surpassing that of graphite, attributed to the material's large interlayer distance and active sites (Fig. 4D). The findings highlight Janus graphene's unique capacity for Na⁺ intercalation, not mirrored in materials like reduced graphene oxide.

Visualization of Na⁺ Intercalation in Janus Graphene Using Operando IES. Operando imaging ellipsometry (IES) was utilized to microscopically observe Na⁺ intercalation in AB graphene films. The technique's sensitivity to refractive index and layer thickness changes was evident in the Δ and Ψ angle variations, directly linked to Na⁺ intercalation, as shown by color transitions in microscopic maps and consistent with CV data (Fig. 5). Increased optical density from Na⁺ accumulation between layers suggested alterations in the material's optoelectronic properties. To corroborate these observations, we employed Park Systems Accurion's optical modeling software (nanofilm_EP4) for a simplified simulation, reflecting the impact of Na⁺ on the ellipsometric angles Δ. The simulation, while qualitative, aligned with the experimental data, attributing the observed IES changes to refractive index variations due to Na⁺ integration within the graphene layers. This macroscopic visualization indicated intercalation starting predominantly from the sample edges, highlighting Janus graphene's integrity and its edge-minimized design compared to more disordered materials, facilitating a nuanced understanding of Na⁺ interaction within the material.

DISCUSSION

The unique structure of Janus graphene, with functionalization on only one surface, offers distinct advantages over symmetrically functionalized graphene. This asymmetric functionalization leads to better alignment in stacked layers due to reduced interlayer interaction, enabling a deeper understanding of Na⁺ intercalation in graphite-based materials. Janus graphene's uniform pore size and minimal edges, resulting from using large-scale CVD graphene sheets, contribute to its superior Na⁺ stabilization compared to pristine graphite, as shown by DFT calculations. Unlike Li+ in crystalline graphite, Na⁺ in Janus graphene undergoes direct stage 1 intercalation without intermediate stages, challenging the classical intercalation model. This behavior, distinct from that in disordered carbons like reduced graphene oxide, is influenced by the presence of AB molecules, facilitating Na⁺ intercalation not observed in pristine graphene. The AB molecules not only act as spacers but also have a synergistic effect with graphene to stabilize Na⁺ ions, as corroborated by DFT studies and operando experiments (Raman, CV, IES) during SIB charging/discharging processes. These findings highlight Janus graphene's potential in SIB applications, demonstrating how asymmetric functionalization can enhance ion storage and stability, offering a new perspective for developing advanced graphite based materials for energy storage.

MATERIALS AND METHODS

Synthesis of NB-functionalized CVD Graphene: CVD graphene was sourced from Graphenea, Spain, and 4-NBD from Sigma-Aldrich or synthesized as per protocol [43]. Functionalization involved immersing graphene in 4-NBD solutions of varying concentrations with sodium dodecyl sulfate for improved solubility. The immersion time varied to control NB group density, followed by extensive washing. Functionalization was selectively done on the top surface, preserving the bottom surface attached to the copper, which was later etched away without affecting the bottom graphene layer.

Electrochemical Conversion to AB Groups: The NB groups were electrochemically reduced to AB groups in a KCl and water/ethanol solution, monitored via CV curves, with a significant reduction peak indicating successful conversion. This process used a three-electrode system, cycling the material from −0.3 to −1.3 V, showing almost complete conversion after the first cycle, followed by thorough rinsing.

Assembly of Single-Graphene Sheets to Prepare Stacked Multilayers: We developed an alternative graphene transfer method to avoid PMMA contamination, using a single PMMA coating on the first graphene layer and then "fishing" additional layers from water after copper etching. This method allowed for the assembly of multiple graphene layers without PMMA residues. The functionalization increased graphene's hydrophilicity, improving layer alignment and stack quality, evidenced by AFM imaging, in contrast to the poor stacking of non-functionalized graphene. This technique ensures better interlayer sliding and high-quality multilayer graphene films, suitable for various substrates and Raman characterization.

Stacking CVD Graphene to Prepare Pristine CVD Graphene Thin Film: Pristine CVD graphene films were stacked using a modified Janus graphene method, adjusting water's surface tension with isopropanol for flatness.

Materials Characterization

IES analysis utilized an IES analysis was conducted with a Park Systems Accurion EP4 system in a polarizer compensator sample analyzer (PCSA) configuration. Unlike traditional ellipsometers, this setup includes an objective and a charge-coupled device (CCD) camera, enabling imaging capabilities. The EP4 features a monochromator that delivers monochromatic light across a broad wavelength spectrum, ranging from 250 to 1700 allowing for versatile material characterization. Material characterizations included XPS (PHI 5500), Raman, TEM, and AFM for surface and thickness analysis. Contact angles were measured using a Theta meter.

Raman spectroelectrochemical characterization was performed with an EL-CELL Raman cell, employing graphene stack working electrodes and lithium and sodium counter electrodes for LIBs and SIBs. Electrolytes of 1 M LiPF6 in EC/ DEC and 1 M NaPF6 in EC/DMC were utilized, with nickel foil for electrical contacts, and a 632-nm laser for targeting the Janus graphene electrode. Na⁺ intercalation/deintercalation was facilitated by CV, Raman-monitored at 20-minute intervals, and capacity was inferred from the fourth cycle's cathodic peak.

DFT calculations, using VASP and PBE with D3 correction, analyzed intercalation energies, optimized graphene lattice constants, and studied interlayer distances, revealing preferred configurations for Li and Na intercalation. Charge transfer analysis indicated ionic bonding between Na and graphene, with charge localization near the cation.

Operando IES explored Na⁺ intercalation dynamics in Janus graphene with a nanofilm_EP4 setup from Park Systems Accurion GmbH, featuring a high-resolution CCD camera and LED light source. Microscopic maps at a 45° incidence angle and 1 µm ellipsometric resolution were recorded, illustrating the sensitivity of IES to Na⁺ intercalation and providing insights into ion transport mechanisms, crucial for optimizing battery material performance.

SUPPLEMENTARY MATERIALS, REFERENCES AND NOTES Supplementary material, references and notes for this article is available at http://advances.sciencemag.org/cgi/content/ full/7/22/eabf0812/DC1