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MXenes, a family of layered transition metal carbides and nitrides, haveshown great promise for use in emerging electrochemical energy storage devices,including batteries and supercapacitors. MXene surfaces are terminated by mixed-O, -F and -OH functional groups as a result of the chemical etching productionprocess. These functional groups are known to be randomly distributed over thesurfaces, with limited experimental control over their composition. There isconsiderable debate regarding the contribution of these functional groups tothe properties of the underlying MXene material. For instance, their measuredLi or Na capacity is far lower than that predicted by theoretical simulations,which generally assume uniformly terminated surfaces. The extent to which thisstructural simplification contributes to such discrepancies is unknown. Weaddress this issue by employing first-principles calculations to compare thestructural, electronic and electrochemical properties of two common MXenes,namely Ti3C2Tx and V2CTx, with both uniform terminating groups and explicitlymixed terminations. Weighted averages of uniformly-terminated layer propertiesare found to give excellent approximations to those of more realistic,randomly-terminated structures. The sodium storage capacity and volume changeduring sodiation in the interlayer space of these MXenes with mixed surfaceterminations are also investigated. The redox reaction is shown to be confinedto the terminating groups for low concentrations of intercalated Na, with theoxidation state of the metal atoms unaffected until higher concentrations ofintercalated Na are achieved. Finally, the average open circuit voltage isshown to be very similar for both Ti3C2T2Na and V2CT2Na with mixedterminations, although it is highly sensitive to the particular composition ofthe terminating groups.

Effect of mixed surface terminations on the structural and electrochemical properties of two-dimensional Ti3C2T2 and V2CT2 MXenes multilayers