Surface Charge Engineering for Covalently Assembling Three-Dimensional MXene Network for All-Climate Sodium Ion Batteries.

MXenes, as excellent candidate anode materials for sodium ion batteries (SIBs), suffer from the sluggish ion-diffusion kinetics resulting from the anchoring effect of the negatively-charged functional groups on their surface on sodium ions. Herein, we introduce positively-charged conductive polyaniline (PANI) to induce self-assembly of Ti3C2Tx MXenes into three-dimensional PANI/Ti3C2Tx network. In this PANI/Ti3C2Tx network, PANI not only intercalates into Ti3C2Tx nanosheets to enlarge the interlayer spacing, but also promotes negative-to-positive transition of the surface charges of the Ti3C2Tx nanosheets, significantly improving ion-diffusion kinetics. Electrochemical test results further confirm the superb ion-diffusion kinetics of PANI/Ti3C2Tx network. Meanwhile, a covalent interaction (Ti-N) between PANI and Ti3C2Tx, proved by X-ray photoelectron spectra (XPS) and X-ray absorption near-edge structure (XANES) tests, plays a key role in stabilizing this network structure. Therefore, PANI/Ti3C2Tx exhibits excellent sodium storage performances with a high specific capacity, superior rate performance and ultra-long lifespan at high current density. More importantly, when operated at the rigorous temperatures from 50 oC to -30 oC, PANI/Ti3C2Tx also exhibits good electrochemical performances. The present work presents a simple strategy for designing 3D porous MXene-based materials to realize high rate performance and all-climate energy storage device.