Unlocking Ultrahigh Initial Coulombic Efficiency of MXene Anode via Presodiation and Electrolyte Optimization.

The low initial Coulombic efficiency (ICE) greatly hinders the practical application of MXenes in sodium-ion batteries. Herein, theoretical calculations confirm that -F and -OH terminations as well as the tetramethylammonium ion (TMA + ) intercalator in sediment Ti 3 C 2 T x (s-Ti 3 C 2 T x ) MXene possess strong interaction with Na + , which impedes Na + desorption during the charging process and results in low ICE. Consequently, Na + -intercalated sediment Ti 3 C 2 T x (Na-s-Ti 3 C 2 T x ) is constructed through Na 2 S·9H 2 O treatment of s-Ti 3 C 2 T x . Specifically, Na + can first exchange with TMA + of s-Ti 3 C 2 T x and then combine with -F and -OH terminations, thus leading to the elimination of TMA + and preshielding of -F and -OH. As expected, the resulting Na-s-Ti 3 C 2 T x anode delivers considerably boosted ICE values of around 71% in carbonate-based electrolytes relative to s-Ti 3 C 2 T x . Furthermore, electrolyte optimization is employed to improve ICE, and the results demonstrate that an ultrahigh ICE value of 94.0% is obtained for Na-s-Ti 3 C 2 T x in the NaPF 6 -diglyme electrolyte. More importantly, Na-s-Ti 3 C 2 T x exhibits a lower Na + migration barrier and higher electronic conductivity compared with s-Ti 3 C 2 T x based on theoretical calculations. In addition, the cyclic stability and rate performance of the Na-s-Ti 3 C 2 T x anode in various electrolytes are comprehensively explored. The presented simple strategy in boosting ICE significantly enhances the commercialization prospect of MXenes in advanced batteries.