Halogenated Ti₃C₂ MXenes with Electrochemically Active Terminals for High-Performance Zinc Ion Batteries

The class of two-dimensional metal carbides and nitrides known as MXenes offer a distinct manner of property tailoring for a wide range of applications. The ability to tune the surface chemistry for expanding the property space of MXenes is thus an important topic, although experimental exploration of surface terminals remains a challenge. Here, we synthesized Ti₃C₂ MXene with unitary, binary, and ternary halogen terminals, e.g., -Cl, -Br, -I, -BrI, and -ClBrI, to investigate the effect of surface chemistry on the properties of MXenes. The electrochemical activity of Br and I elements results in the extraordinary electrochemical performance of the MXenes as cathodes for aqueous zinc ion batteries. The -Br- and -I-containing MXenes, e.g., Ti₃C₂Br₂ and Ti₃C₂I₂, exhibit distinct discharge platforms with considerable capacities of 97.6 and 135 mAh·g^-1. Ti₃C₂(BrI) and Ti₃C₂(ClBrI) exhibit dual discharge platforms with capacities of 117.2 and 106.7 mAh·g^-1. In contrast, the previously discovered MXenes Ti₃C₂Cl₂ and Ti₃C₂(OF) exhibit no discharge platforms and only ∼50% of capacities and energy densities of Ti₃C₂Br₂. These results emphasize the effectiveness of the Lewis-acidic-melt etching route for tuning the surface chemistry of MXenes and also show promise for expanding the MXene family toward various applications.