Facile Synthesis of Fluoride-Free MXene Nanosheets as an Efficient Anode for Lithium-Ion Capacitor

MXenes are a promising class of two-dimensional (2D) nanomaterials known for their exceptional metallic conductivity and adjustable surface chemistry. However, the current state-of-the-art synthesis methods rely on the chemical etching of MAX phase (e.g., Ti₃AlC₂) with HF or fluoride-based compounds, leading to fluorine-terminated MXenes. These MXenes suffer from poor stability in ambient conditions, restricting their applications, particularly in lithium-ion-based batteries and capacitors (LIBs and LICs). In this study, we present a two-step method to produce fluorine-free MXene, addressing the stability issues of MXene in aqueous dispersions and relatively improved performance in LICs. Specifically, an efficient etching process employing hydroiodic acid (HI) with vinegar is used for the selective removal of the A layer from the MAX phase, resulting in F-free exfoliated MXenes (HVM). The HVM shows an outstanding electrical conductivity of 388 S cm^-1, maintaining high stability in aqueous dispersions over two weeks. HVM as electrode shows significantly enhanced Li⁺ ion storage capabilities, delivering a discharge capacity of 295 mAh g^-1 over 500 cycles at 1 A g^-1, substantially outperforming MXenes derived from F-based etching approaches. Furthermore, the HI-vinegar etching mechanism introduces unique surface functionalities that provide HVM superior cycling stability and rate capability, enabling more stable, high-performance MXene-based energy storage devices.