Nanoscale Analysis Sheds Light on Advanced Energy Storage Material

 Nanoscale Analysis Sheds Light on Advanced Energy Storage Material

Researchers at the Department of Energy’s (DOE) Oak Ridge National Laboratory (ORNL) have used advanced in-situ microscopy and theoretical calculations to better understand the properties of a promising next-generation energy storage material for chemical batteries.
ORNL’s Fluid Interface Reactions, Structures, and Transport (FIRST) team observed how ions move and diffuse between layers of a two-dimensional electrode during electrochemical cycling. Understanding this migration is critical to understanding how energy is stored in a material called MXene and how this material achieves such exceptional energy storage properties.
MXene acts as a two-dimensional electrode that could be fabricated with the flexibility of a sheet of paper. It is based on MAX-phase ceramics, which have been studied for decades. Chemical removal of the material’s “A” layer leaves two-dimensional flakes composed of transition metal layers, denoted in the material’s name by the letter “M”. These flakes sandwich carbon or nitrogen layers, denoted by the “X”. The resulting MXene physically resembles graphite.
MXene’s energy-storage properties had previously been observed on a microscopic scale, but less was understood about the properties at the nanoscale. MXene’s very high capacitance has only recently been explored as an energy-storage medium for advanced batteries. Important to the material’s energy storage performance are the interaction and charge transfer of the ion and the MXene layers. The adsorption processes drive interesting phenomena which govern these processes.
FIRST researchers explored how the ions entered the material, and how they moved once inside that material. If positively-charged cations are introduced into the negatively-charge MXene, the material contracts and becomes stiffer. The researchers measured the local changes in this stiffness and found a direct correlation between the diffusion pattern of the ions and the material’s stiffness.
These studies require the ions to be introduced into the electrode in a solution, which means researchers must work in a liquid environment, the first time such experiments have been conducted in this way. Researchers can then measure the mechanical properties of the ions and the MXene in-situ and at different stages of charge storage. This gives them insight into where the ions are stored in the material.
A nanoscale study of the ionic interactions within the electrode had not been possible prior to the team’s research. These studies emphasize the need for in-situ analysis to understand nanoscale elastic changes in the two-dimensional material in dry and wet environments, and the effect of ion storage on the energy-storage material over time.
With greater understanding of the process’s fundamental mechanical properties, researchers hope to tune MXene’s energy storage properties and improve the material’s performance and lifespan. Next steps include improving the material’s ionic diffusion paths and exploring other materials in the MXene family.
The work was recently published in the journal Advanced Energy Materials.

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