Ammonia-Assisted Nb2CTx MXenes with Tuneable Layering and Surface Chemistry for Superior Li and Na-Ion Energy Storage

Abstract

MXenes are widely recognized for battery applications due to their rapid ion-intercalation capability and excellent cycling stability. However, conventional synthesis methods often involve hazardous HF, prolonged reaction times, and complex procedures, limiting their practical implementation. Here, we present an efficient and sustainable ammonia-assisted hydrothermal strategy for synthesizing Nb2CTx MXene from the Nb2AlC MAX phase using a mild etchant, providing a safe, cost-effective, and scalable route. The as-prepared MXene was further delaminated via probe sonication, yielding an expanded interlayer spacing of ~9.7 Å and optimized surface functionalities. XPS reveals an oxygen-rich (-O/-OH) surface that promotes Li⁺/Na⁺ interaction and ion diffusion, consistent with the enhanced reversibility observed in CV and GCD profiles. As an anode, Nb2CTx delivers a high reversible discharge capacity of 484 mAh g⁻¹ at 0.1 A g⁻¹ with stable cycling over 100 cycles in lithium-ion batteries, and 198 mAh g⁻¹ at 0.1 A g⁻¹ over 100 cycles in sodium-ion batteries. Ex-situ post-mortem XRD confirms that Nb2CTx retains structural integrity and electrochemical stability after 100 cycles. Importantly, hydrothermal etching eliminates concentrated HF while producing well-exfoliated flakes. Combined structural, surface, and electrochemical analyses reveal the origins of its performance, providing a blueprint for next-generation sustainable MXene electrodes.