Enhancing Lithium Adsorption and Diffusion toward Extraordinary Lithium Storage Capability of Freestanding Ti3C2Tx MXene

Chemical etching method shows potential for large-scale and low-cost processed MXenes but incorporates surface terminations such as F&OH that probably deteriorate the lithium storage characteristics. Herein, we propose that tailoring appropriate surface functionalization and the intrinsic electrical properties can dramatically enhance the lithium storage capability of Ti3C2Tx (T stands for F, OH, and O) MXene materials. By carefully controlling the annealing process, the Ti3C2Tx films possess fewer F&OH elements so that with higher conductivity, they are still freestanding and flexible. Density functional theory computations of the low-F&OH-containing Ti3C2Tx show low-ion-diffusion barrier values of 0.34-0.43 eV and a significant increase of Li adsorption energy by 6-30 times compared to those of high-F&OH-containing Ti3C2Tx, suggesting high Li-ion storage and transfer capabilities can be achieved in low-F&OH-containing Ti3C2Tx MXenes. Dramatically, the heteroatom-controlled Ti3C2Tx MXene films show reversible capacities of 221 mAh g(-1) for lithium-ion batteries at a current density of 0.1 C (1 C = 320 mAh g(-1)), which is the highest up to now for pure Ti-based MXenes. These results demonstrate the importance of surface chemistry and electronic structure of MXene in the lithium storage capability, which provides valuable information on designing high-performance MXene-based materials for energy storage, optoelectronic, thermoelectric, and magnetic applications.