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 Ti₃C₂T (s-Ti₃C₂T) MXene possess strong interaction with Na⁺, which impedes Na⁺ desorption during the charging process and results in low ICE. Consequently, Na⁺-intercalated sediment Ti₃C₂T (Na-s-Ti₃C₂T) is constructed through Na₂S·9H₂O treatment of s-Ti₃C₂T. Specifically, Na⁺ can first exchange with TMA⁺ of s-Ti₃C₂T 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-Ti₃C₂T anode delivers considerably boosted ICE values of around 71% in carbonate-based electrolytes relative to s-Ti₃C₂T. 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-Ti₃C₂T in the NaPF₆-diglyme electrolyte. More importantly, Na-s-Ti₃C₂T exhibits a lower Na⁺ migration barrier and higher electronic conductivity compared with s-Ti₃C₂T based on theoretical calculations. In addition, the cyclic stability and rate performance of the Na-s-Ti₃C₂T anode in various electrolytes are comprehensively explored. The presented simple strategy in boosting ICE significantly enhances the commercialization prospect of MXenes in advanced batteries.