INTERLAYER SPACING AND SURFACE CHEMISTRY ENGINEERING OF TWO-DIMENSIONAL ELECTRODE MATERIALS FOR ENHANCED ION TRANSPORT IN FLEXIBLE SUPERCAPACITORS AND SODIUM-ION BATTERIES

Abstract

Two-dimensional electrode materials, including MXenes, molybdenum disulphide nanosheets, and reduced graphene oxide, offer high theoretical surface areas and tunable surface chemistries that make them attractive candidates for next-generation flexible supercapacitors and sodium-ion batteries. However, their practical electrochemical performance is constrained by restacking-induced interlayer collapse, which reduces accessible ion-transport pathways, and by an incomplete understanding of how specific surface functional groups regulate ion adsorption energetics, diffusion behavior, and charge-storage mechanisms at the electrode–electrolyte interface. This study presents a systematic investigation of interlayer-spacing and surface-chemistry engineering strategies across three two-dimensional electrode material families: Ti₃C₂Tₓ MXene, MoS₂ nanosheets, and nitrogen-doped reduced graphene oxide. The independent and combined effects of molecular intercalation, chemical etching conditions, and functional-group modification were evaluated in terms of sodium-ion diffusion coefficients, areal capacitance, rate capability, cycling stability, and flexible-device performance. Interlayer-spacing expansion of Ti₃C₂Tₓ MXene from 1.18 to 2.14 nm through DMSO intercalation increased the sodium-ion diffusion coefficient by 3.4-fold and improved areal capacitance from 312 to 487 mF cm⁻² at 2 mA cm⁻². Oxygen-functional-group-enriched MoS₂ achieved a reversible sodium-ion storage capacity of 384 mAh g⁻¹ at 0.1 A g⁻¹ with 89.4% capacity retention after 1,000 cycles. Flexible supercapacitor cells assembled on polyimide substrates maintained 94.2% capacitance retention after 5,000 bending cycles at 180°, confirming strong mechanical durability for wearable energy-storage applications. Density functional theory calculations further indicated that hydroxyl-dominant MXene surfaces provide sodium-ion adsorption energetics approximately 0.38 eV more favorable than fluorine-dominant surfaces, consistent with the experimentally observed 2.1-fold enhancement in sodium-ion diffusion. These findings establish quantitative structure–property–performance relationships that provide practical design rules for simultaneously optimizing ion transport, electrochemical performance, cycling stability, and mechanical flexibility in next-generation sodium-ion energy-storage devices.