Molecular-Level Interfacial Chemistry Regulation of MXene Enables Energy Storage beyond Theoretical Limit.

TiCT MXene often suffers from poor lithium storage behaviors due to its electrochemically unfavorable OH terminations. Herein, we propose molecular-level interfacial chemistry regulation of TiCT MXene with phytic acid (PA) to directly activate its OH terminations. Through constructing hydrogen bonds (H-bonds) between oxygen atoms of PA and OH terminations on TiCT surface, interfacial charge distribution of TiCT has been effectively regulated, thereby enabling sufficient ion-storage sites and expediting ion transport kinetics for high-performance energy storage. The results show that Li ions preferably bind to H-bond acceptors (oxygen atoms from PA), and the flexibility of H-bonds therefore renders their interactions with adsorbed Li ions chemically "tunable", thus alleviating undesirable localized geometric changes of the OH terminations. Meanwhile the H-bond-induced microscopic dipoles can act as directional Li-ion pumps to expedite ion diffusion kinetics with lower energy barrier. As a result, the as-designed TiCT/PA achieves a 2.4-fold capacity enhancement compared with pristine TiCT (even beyond theoretical capacity), superior long-term cyclability (220.0 mAh g after 2000 cycles at 2.0 A g), and broad temperature adaptability (-20 to 50 °C). This work offers a promising interface engineering strategy to regulate microenvironments of inherent terminations for breaking through the energy storage performance of MXenes.