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Ti2CT2 MXene as Anodes for Metal Ion Batteries: From Monolayer to Bilayer to Pillar Structure.

The electrochemical performances of Ti2CT2 (T = F, O, and OH) MXenes with different layer structures (monolayer, bilayer, and pillared structures) as anodes for mono-/multivalent metal ion (Li+, Na+, Mg2+, and Al3+) batteries (MIBs) were studied via first-principles simulations. First, metal ions (MIs) adsorbed on Ti2CT2 monolayers were investigated to reveal the influence of MXene terminated groups on MIB performance. This indicated that O-terminated MXenes would be more suitable as electrodes. In particular, the theoretical capacity of Mg2+ on a Ti2CO2 monolayer could be more than 1500 mA h g-1. Then, MIs intercalated into MXene bilayers were considered to better understand the charging/discharging mechanism. In a Ti2CO2 bilayer with larger interlayer spacing, monovalent MIs and Mg2+ could form a multilayer accompanied by drastic expansion/contraction of the electrode, which still needs to be solved. Finally, imidazolium-based ionic liquids were used to preintercalate into MXene due to the matching size of the imidazolium cation, which effectively improved MXene stability and inhibited the self-stacking of layered MXenes. Our research would be helpful for theoretically regulating MXene functional groups and adjusting the interlayer spacing of MXenes via selecting guest molecules for designing MIBs and other energy storage devices.