Sulfonic-Group-Grafted TiCT MXene: A Silver Bullet to Settle the Instability of Polyaniline toward High-Performance Zn-Ion Batteries.

Polyaniline (PANI) is a promising cathode material for Zn-ion batteries (ZIBs) due to its intrinsic conductivity and redox activity; however, the achievements of PANI in high-performance ZIBs are largely hindered by its instability during the repeated charge/discharge. Taking advantage of the high conductivity, flexibility, and grafting ability together, a surface-engineered TiCT MXene is designed as a silver bullet to fight against the deprotonation and swelling/shrinking issues occurring in the redox process of PANI, which are the origins of its instability. Specifically, the sulfonic-group-grafted TiCT(S-TiCT) continuously provides protons to improve the protonation degree of PANI and maintains the polymer backbone at a locally low pH, which effectively inhibits deprotonation and brings high redox activity along with good reversibility. Meanwhile, the conductive and flexible natures of S-TiCT assist the fast redox reaction of PANI and concurrently buffer its corresponding swelling/shrinking. Therefore, the S-TiCT-enhanced PANI cathode simultaneously achieves a high discharge capacity of 262 mAh g at 0.5 A g, a superior rate capability of 160 mAh g at 15 A g, and a good cyclability over 5000 cycles with 100% coulombic efficiency. This work enlightens the development of versatile MXene surface engineering for advanced batteries.