These MXenes, which have exhibited very high capacitance, or ability to store electrical charge, have only recently been explored as an energy storage medium for advanced batteries.

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.

It is reported here with in situ imaging, combined with density functional theory of the elastic changes of a 2D titanium carbide (Ti₃C₂T_x) based electrode in direction normal to the basal plane (electrode surface) during alkaline cation intercalation/extraction. … The results show a strong correlation between the cations content and the out-of-plane elastic modulus. This strategy enables identifying the preferential intercalation pathways within a single particle, which is important for understanding ionic transport in these materials.

—Come et al.

ORNL’s Fluid Interface Reactions, Structures and Transport (FIRST) research team, using scanning probe microscopy made available through the Center for Nanophase Materials Sciences (CNMS) user program, have observed for the first time at the nanoscale and in a liquid environment how ions move and diffuse between layers of a MXene electrode during electrochemical cycling.

This migration is critical to understanding how energy is stored in the material and what drives its exceptional energy storage properties.

Nina Balke, one of the ORNLteam of researchers working with Drexel University’s Yury Gogotsi in the FIRST Center, a DOE Office of Science Energy Frontier Research Center, said that the new technnique allows the tracking of how ions enter the interlayer spaces. There is very little information on how this actually happens, she said.

The energy storage properties have been characterized on a microscopic scale, but no one knows what happens in the active material on the nanoscale in terms of ion insertion and how this affects stresses and strains in the material.

—Nina Balke

The interaction and charge transfer of the ion and the MXene layers is very important for its performance as an energy storage medium. The adsorption processes drive interesting phenomena that govern the mechanisms we observed through scanning probe microscopy.

—FIRST researcher Jeremy Come

The researchers explored how the ions enter the material, how they move once inside the materials and how they interact with the active material. For example, if cations, which are positively charged, are introduced into the negatively charged MXene material, the material contracts, becoming stiffer.

When a negative bias is applied to a two-dimensional MXene electrode, Li+ ions from the electrolyte migrate in the material via specific channels to the reaction sites, where the electron transfer occurs. Scanning probe microscopy at Oak Ridge National Laboratory has provided the first nanoscale, liquid environment analysis of this energy storage material. Source: ORNL. Click to enlarge.

That observation laid the groundwork for the scanning probe microscopy-based nanoscale characterization. The researchers measured the local changes in stiffness when ions enter the material. There is a direct correlation with the diffusion pattern of ions and the stiffness of the material.

Come noted that the ions are inserted into the electrode in a solution.

Therefore, we need to work in liquid environment to drive the ions within the MXene material. Then we can measure the mechanical properties in-situ at different stages of charge storage, which gives us direct insight about where the ions are stored.

—Jeremy Come

Until this study the technique had not been done in a liquid environment.

The processes behind ion insertion and the ionic interactions in the electrode material had been out of reach at the nanoscale until the CNMS scanning probe microscopy group’s studies. The experiments underscore the need for in-situ analysis to understand the nanoscale elastic changes in the 2D material in both dry and wet environments and the effect of ion storage on the energy storage material over time.

The researchers’ next steps are to improve the ionic diffusion paths in the material and explore different materials from the MXene family. Ultimately, the team hopes to understand the process’s fundamental mechanism and mechanical properties, which would allow tuning the energy storage as well as improving the material’s performance and lifetime.

ORNL’s FIRST research team also provided additional calculations and simulations based on density functional theory that support the experimental findings. The work was recently published in the Journal Advanced Energy Materials.

The research team in addition to Balke and Come and Drexel’s Gogotsi included Michael Naguib, Stephen Jesse, Sergei V. Kalinin, Paul R.C. Kent and Yu Xie, all of ORNL.

The FIRST Center is an Energy Frontier Research Center supported by the DOE Office of Science (Basic Energy Sciences). The Center for Nanophase Materials Sciences and the National Energy Research Scientific Computing Center are DOE Office of Science User Facilities.

Resources

• Come, J., Xie, Y., Naguib, M., Jesse, S., Kalinin, S. V., Gogotsi, Y., Kent, P. R. C. and Balke, N. (2016) “Nanoscale Elastic Changes in 2D Ti₃C₂T_x (MXene) Pseudocapacitive Electrodes.” Adv. Energy Mater., 1502290 doi: 10.1002/aenm.201502290