Few-layer large Ti3C2Tx sheets exfoliated by NaHF2 and applied to the sodium-ion battery

Since the original discovery, MXene can be synthesized by wet-chemical etching from the MAX phase. To avoid the shortcomings of the previous etchant, NaHF2 aqueous solution was used for the selective removal of Al from Ti3AlC2, while intercalating Na+ between layers. The d-spacing of the resulting accordion-like Ti3C2Tx is up to 12.35 A; the by-product Na3AlF6 can be removed by H2SO4, and the product yield can reach about 85%. After a short time of centrifugation and sonication, few-layer (10–20 layers) Ti3C2Tx sheets in a large lateral size (3–4 μm) can be prepared, with high conductivity up to 2.3 × 105 S m−1. Ti3C2Tx can be stably dispersed in water and organic solvents for at least 96 h, and has excellent anti-thermal-oxidation ability. As the anode for sodium ion batteries, the Ti3C2Tx electrode delivers a high reversible capacity of 70 mA h g−1 after 900 cycles at a current density of 1 A g−1 with an extremely low-capacity decay rate. From 900 to 1000 cycles, the capacity can even be increased to 130 mA h g−1, and the Coulomb efficiency is close to 100%, showing that MXene can provide a stable structure for the Na+ cycle.