Enhanced Functional Properties of Ti3C2Tx MXenes as Negative Electrodes in Sodium-Ion Batteries by Chemical Tuning

The large implementation of electrochemical energy storage devices requires the development of new chemistries tailored for specific uses. Sodium‐ion batteries (SIBs) can cover different application fields, however the state‐of‐the‐art negative material, hard carbon, suffers from poor cyclability and rate capability. MXenes are a vast class of 2D‐materials of the general formula Mn +1Xn Tx (M = transition metal, X = C or N, and T = M‐terminating group) with peculiar structural features able to reversible intercalate chemical species, such as alkaline cations. The MXene compound Ti3C2Tx is one of the most investigated, thanks to the easy preparation route by etching of the pristine compound Ti3AlC2. In this work, the effect of the etching conditions and of the postsynthesis thermal treatments on the chemical, morphological, and structural properties of Ti3C2Tx are investigated, and in turn the correlation between its features and the functional properties as negative materials in SIBs are studied. The Ti3C2Tx obtained in high hydrofluoric concentration and after a 300 °C thermal treatment shows 110 mAh g−1 at 30 mA g−1 with an average potential of 1.33 V versus Na+/Na, 100% and good rate capability, since it is still able to deliver 73 mAh g−1 at 1500 mA g−1.