Probing the Mechanical Deformation of 2D Titanium Carbide (MXene) upon Cation Intercalation at the Nanoscale

The titanium carbide Ti₃C₂T_x (where T_x stands for termination functions at the surface such as –OH, =O, and –F) is a member of the recently discovered family of two-dimensional (2D) materials known as MXenes. This material demonstrates a high intercalation capacitance related to the rapid transport of ions within the structure, making this a promising electrode material for supercapacitors.[1] To date however, the intercalation mechanism is poorly understood and other techniques able to probe the dynamics are required. In this work, in-situ Atomic Force Microscopy is used to monitor the strain developed in a Ti₃C₂T_x electrode during intercalation/extraction of monovalent and divalent cations in a variety of aqueous electrolytes. Interestingly, the electrode undergoes a large contraction during Li⁺, Na⁺ or Mg²⁺ intercalation, differentiating the MXene paper from conventional electrodes where redox intercalation of ions (e.g. Li⁺) into the bulk phase (e.g. graphite, silicon) results in volumetric expansion. The relative deformation amplitude strongly depends on the cation radius and electric charge ranging from almost no change to over 15% of shrinkage.[2] This feature may explain the excellent rate performance and cyclability reported for MXenes.The unique mechanical changes induced by the presence of cations between the layers were investigated by Contact Resonance Piezoresponse Force Microscopy. Spatial mapping of the resonance frequency with high resolution showed that the elastic modulus of the carbide increases when Li⁺ ions are intercalated, in good agreement with the strong interactions between MXene sheets that leads to contraction. Furthermore, it revealed that the cation intercalation preferentially occurs at the shallow sites of the MXene flakes.

These results are exciting because they shed light on the intricate interplay of the MXene mechanical properties with the electrochemical performance. Moreover, they show that the cation dynamics in the confined 2D spaces can be efficiently probed with in-situ AFM techniques.

References

[1] M. Ghidiu et al., Conductive Two-Dimensional Titanium Carbide ‘Clay’ with High Volumetric Capacitance, Nature, 2014, 516, 78-82

[2] J. Come et al., Controlling the Actuation Properties of MXene Paper Electrodes upon Cation Intercalation, Nano Energy, under review.