Nanoscale Mechanical Changes of 2D Titanium Carbide (MXene) Pseudocapacitive Electrodes upon Cation Intercalation

Designing sustainable electrodes for next generation energy storage devices relies on the understanding of their fundamental properties at the nanoscale, including the comprehension of ions insertion into the electrode and their interactions with the active material. One consequence of ion storage is the change in the electrode volume resulting in mechanical strain and stress that can strongly affect the cycle life. Therefore, it is important to understand the changes of dimensions and mechanical properties occurring during electrochemical reactions. While the characterization of mechanical properties via macroscopic measurements is well documented, in-situ characterization of their evolution has never been achieved at the nanoscale. Two dimensional (2D) carbides, known as MXenes, are promising materials for supercapacitors and various kinds of batteries, as they demonstrate a high intercalation capacitance related to the rapid transport of ions within the structure [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 Ti 3 C 2 T 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 Mg 2+ intercalation, differentiating the Ti 3 C 2 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] . The unique mechanical changes induced by the presence of cations between the layers were investigated by Contact Resonance Atomic Force Microscopy [3] . Spatial mapping of the resonance frequency with high resolution showed that the elastic modulus of the carbide in the direction normal to the basal plane increases of 25 GPa when Li + ions are intercalated, in good agreement with the strong interactions between MXene sheets that leads to contraction. However, no changes in the elasticity were observed during the intercalation of larger K + ions. Furthermore, these measurements 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 by controlling the solid/liquid interface. Moreover, they show that the cation dynamics in the confined 2D spaces can be efficiently probed with in-situ AFM techniques. The experiments and sample preparation in this work were supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. The facilities to perform the experiments were provided by the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. [1] M. Ghidiu et al., Nature 2014 , 516, 78-82. [2] J. Come et al., Nano Energy 2015 , 17, 27-35. [3] J Come et al, submitted.