3D Honeycomb‑Like SnS₂ Quantum Dot / rGO Composites: A High‑Performance Anode for Lithium and Sodium‑Ion Batteries

Abstract

Metal dichalcogenides such as SnS₂ suffer from severe capacity fade due to volume pulverization and poor conductivity. We report a scalable spray‑drying and sulfidation route that produces a 3‑D honeycomb‑like reduced graphene oxide (rGO) scaffold uniformly decorated with ~6 nm SnS₂ quantum dots (QDs). The interconnected porous network confines volume changes, provides ample electrolyte reservoir, and dramatically enhances electronic pathways. The resulting 3‑D SnS₂ QDs/rGO electrode delivers 862 mAh g⁻¹ at 0.1 A g⁻¹ after 200 cycles in lithium‑ion batteries (LIBs) and 233 mAh g⁻¹ at 0.5 A g⁻¹ after 200 cycles in sodium‑ion batteries (SIBs). This design offers a viable strategy for next‑generation high‑capacity anodes.

Background

Li‑ion batteries (LIBs) and Na‑ion batteries (SIBs) are pivotal for portable electronics and electric vehicles, yet commercial graphite anodes are limited by a 372 mAh g⁻¹ theoretical capacity and structural instability at high rates. Layered metal dichalcogenides, especially SnS₂, present a theoretical capacity (~1230 mAh g⁻¹) far above graphite but are hindered by large volume changes and low electronic conductivity during lithiation/sodiation. Integrating SnS₂ with conductive, 3‑D porous scaffolds—most notably graphene—can simultaneously buffer expansion, improve electron transport, and prevent particle agglomeration. Recent studies on Co₃O₄, MoS₂, and WS₂ in 3‑D architectures demonstrate enhanced LIB performance, motivating the present investigation.

Methods

Synthesis of Polystyrene Nanospheres

Polystyrene (PS) nanospheres (~200–300 nm) were prepared by emulsion polymerization of styrene in water, followed by freeze‑drying at –50 °C for 24 h. The product was purified by repeated washing with water and ethanol.

Fabrication of 3‑D SnS₂ QDs/rGO Composite

A GO (2.5 wt %) colloid was mixed with PS nanospheres and SnCl₄·5H₂O (1.5 g) under ultrasonication for 1 h. The suspension was spray‑dried (140 °C, 800 mL h⁻¹) to form a porous precursor, which was calcined at 450 °C for 2 h in Ar to remove PS and yield 3‑D SnO₂/rGO. Subsequent sulfidation with thiourea at 350 °C for 12 h in Ar converted SnO₂ to SnS₂, producing the final 3‑D SnS₂ QDs/rGO composite.

Characterization

Structural and compositional analyses were performed by XRD (Cu‑Kα), Raman spectroscopy (532 nm), XPS, TEM/SEM, BET surface area (N₂ adsorption), and TGA (air, 10 °C min⁻¹). Electrochemical tests employed coin‑type cells with Li/Na metal counter electrodes, 1 M LiPF₆/EC:DEC (1:1) for LIBs and 1 M NaClO₄/EC:DEC (1:1) for SIBs. Galvanostatic charge/discharge and cyclic voltammetry were recorded over 0.01–3.00 V.

Results and Discussion

Structural Features

The 3‑D honeycomb architecture is evident in SEM/TEM images, with rGO layers forming continuous walls and ~200–300 nm voids derived from sacrificial PS spheres. HRTEM reveals SnS₂ QDs (~6 nm) uniformly dispersed within the rGO matrix, with lattice spacing of 0.32 nm corresponding to the (100) plane. XRD patterns confirm crystalline SnS₂ (JCPDS 23‑0677) and reduced graphene oxide peaks. BET analysis shows a 21.99 m² g⁻¹ surface area and mesoporous distribution conducive to ion transport.

Electrochemical Performance – LIBs

The composite exhibits an initial discharge capacity of 1400 mAh g⁻¹, surpassing the theoretical SnS₂ capacity due to SEI formation. After 200 cycles at 0.1 A g⁻¹, the capacity remains 870 mAh g⁻¹, while the pure SnS₂ electrode drops to 16 mAh g⁻¹. Rate capability is superior: 870, 770, 622, and 452 mAh g⁻¹ at 0.1, 0.2, 0.5, and 1 A g⁻¹, respectively, with quick recovery upon return to 0.1 A g⁻¹. These results confirm the 3‑D network’s role in mitigating volume change, enhancing conductivity, and preventing SnS₂ aggregation.

Electrochemical Performance – SIBs

In SIBs, the composite delivers 397 mAh g⁻¹ at 0.1 A g⁻¹ and retains 233 mAh g⁻¹ after 200 cycles at 0.5 A g⁻¹, whereas the pure SnS₂ electrode falls to 6 mAh g⁻¹. CV curves show stable, overlapping redox peaks after the first cycle, indicating excellent reversibility. The dominant capacitive contribution (≈76 % at 1 mV s⁻¹) arises from the high surface area and porous structure.

Mechanistic Insights

During lithiation/sodiation, SnS₂ undergoes intercalation, conversion to Li₂S/S₂Na₂, and alloying with Sn to form LiₓSn/NaₓSn. The 3‑D rGO scaffold accommodates these transformations by providing space and maintaining electronic connectivity, thereby sustaining high coulombic efficiency (~99 %) and cycle life.

Conclusions

We have demonstrated a facile, scalable route to 3‑D honeycomb‑like SnS₂ QDs/rGO composites that combine high capacity, robust cycling, and rapid rate performance in both LIBs and SIBs. The ~6 nm QDs uniformly anchored on rGO, together with adjustable pore size via PS sphere dimension, enable effective volume buffering and enhanced conductivity. The electrode achieves 862 mAh g⁻¹ after 200 cycles at 0.1 A g⁻¹ (LIB) and 233 mAh g⁻¹ after 200 cycles at 0.5 A g⁻¹ (SIB), outperforming many reported SnS₂‑based anodes. This architecture offers a promising platform for next‑generation high‑performance anode materials.