When a negative bias is applied to a two-dimensional MXene electrode, lithium ions from the electrolyte migrate in the material via specific channels to the reaction sites, where electron transfer occurs. Scanning probe microscopy at ORNL has provided the first nanoscale, liquid environment analysis of this energy storage material. Image: ORNL.Researchers at the US Department of Energy (DOE)'s Oak Ridge National Laboratory (ORNL) have combined advanced in situ microscopy and theoretical calculations to uncover important clues about the properties of a promising next-generation energy storage material for supercapacitors and batteries.
ORNL's Fluid Interface Reactions, Structures and Transport (FIRST) research team used scanning probe microscopy to observe how ions move and diffuse between layers of a two-dimensional electrode during electrochemical cycling. This is the first time this migration has been studied at the nanoscale and in a liquid environment, and is critical to understanding how energy is stored in the electrode material, called MXene, and what drives its exceptional energy storage properties.
"We have developed a technique for liquid environments that allows us to track how ions enter the interlayer spaces. There is very little information on how this actually happens," said Nina Balke, one of a team of researchers working with Drexel University's Yury Gogotsi in the FIRST Center, a DOE Office of Science Energy Frontier Research Center.
"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," Balke said.
MXene is a two-dimensional material that can possess the flexibility of a sheet of paper; it is based on MAX-phase ceramics, which have been studied for decades. Chemically removing the ‘A’ layer leaves two-dimensional flakes composed of transition metal layers (the ‘M’) that sandwich carbon or nitrogen layers (the ‘X’), with the resulting MXene physically resembling graphite. These MXenes, which exhibit very high capacitance (ability to store electrical charge), have only recently been explored as an energy storage medium for advanced batteries.
"The interaction and charge transfer of the ion and the MXene layers is very important for its performance as an energy storage medium," explained FIRST researcher Jeremy Come. "The adsorption processes drive interesting phenomena that govern the mechanisms we observed through scanning probe microscopy."
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, the material contracts, becoming stiffer.
That observation laid the groundwork for the scanning probe microscopy-based nanoscale characterization, which allowed the researchers to measure the local changes in stiffness when ions enter the material. They found a direct correlation between 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 a 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," he said.
Until this study, scanning probe microscopy had not been conducted in a liquid environment. As a consequence, the processes behind ion insertion and the ionic interactions in the electrode material had been out of reach at the nanoscale. These latest experiments underscore the need for in situ analysis to understand the nanoscale elastic changes in the two-dimensional 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 to explore different materials from the MXene family. Ultimately, they hope to understand the process's fundamental mechanism and mechanical properties, allowing them to tune the energy storage as well as improve 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. This work was recently published in Advanced Energy Materials.
This story is adapted from material from Oak Ridge National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.