Tracking Ion Intercalation into Layered Materials on the Nanoscale

Abstract

In the past decades, electrochemical capacitors (supercapacitors) have drawn considerable attention for applications in portable electronics, grid and transportation systems due to their rapid power delivery and an almost unlimited cycle life. In search for new materials with higher electrochemical performance, two dimensional (2D) transition metal carbides – MXenes, are of particular interest owing to their excellent electrical conductivity and high volumetric capacitance[1] [2] . A variety of aqueous cations can be electrochemically intercalated into Ti3C2, the most studied MXene to date, resulting in very high volumetric capacitance outperforming a variety of carbons[1] . The mechanism for high capacitance was essentially described as intercalation pseudo-capacitance arising from redox reactions of the Ti atoms. On the other hand, Ti3C2shows intercalation capacitive behavior even at quite high rates, while normally a slow intercalation takes place in layered battery materials. Similar to graphite or other electrode materials in various electrolytes, MXene electrodes also show a significant change in volume when ions are intercalated. This electro-chemo-mechanical coupling can be used to get unprecedented insight into ion intercalation pathways with lateral resolution of 10’s of nm using Scanning Probe Microscopy (SPM) techniques. In this communication, we introduce contact resonance SPM which allows to extract mechanical properties and its changes under electrochemical control when ions are intercalated into MXene[3] [4] . Of special interest to boost energy storage is the intercalation of multivalent ions such as Mg2+ which suffers from sluggish intercalation and transport kinetics due to its ion size. By combining traditional electrochemical characterization techniques with electrochemical dilatometry and contact resonance atomic force microscopy, the synergetic effects of pre-intercalation of K+ ions are demonstrated to improve charge storage of multivalent ions, as well as tune mechanical and actuation properties of Ti3C2 MXene[4] [5] .Our results have important implications for quantitatively understanding the charge storage processes in intercalation compounds and provide a new path for studying the mechanical evolution of energy storage materials. 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.