Increasing Energy Density of MXenes Could Improve Supercapacitors

March 04, 2019 by Scott McMahan

Scientists at the Department of Energy's Oak Ridge National Laboratory, as well as Drexel University, and their partners have found a way to increase the energy density of promising energy-storage materials, conductive two-dimensional ceramics called MXenes. (See layers of MXenes in an electrode above).

Today's batteries, which rely on charges stored in the bulk of their electrodes, offer high energy-storage capacity. However, slow charging speeds can limit their application in consumer electronics and electric vehicles.

One possible solution may be electrochemical capacitors, known as supercapacitors, which can store charge at the surface of the electrode material for fast charging and discharging. So far, however, supercapacitors don't have the energy density or charge-storage capacity of batteries.

"The energy storage community is conservative, using the same few electrolyte solvents for all supercapacitors," said principal investigator Yury Gogotsi, a Drexel University professor who planned the study with his postdoctoral researcher Xuehang Wang. "New electrode materials like MXenes require electrolyte solvents that match their chemistry and properties."

The surfaces of various MXenes can be covered with a variety terminal groups, including oxygen, hydroxyl, or fluorine species, which interact strongly and specifically with different solvents and dissolved salts in the electrolyte. In this way, a good electrolyte solvent-electrode match may boost charging speed or increase storage capacity.

"Our study showed that the energy density of supercapacitors based on two-dimensional MXene materials can be significantly increased by choosing the appropriate solvent for the electrolyte," added co-author Lukas Vlcek of the University of Tennessee, who conducts research in UT and ORNL's Joint Institute for Computational Sciences. "By simply changing the solvent, we can double the charge storage."

The work was conducted as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center led by ORNL and supported by the DOE's Office of Science. FIRST research studies fluid-solid interface reactions. The findings can have consequences for energy transport in common applications.

Drexel's Ke Li synthesized the titanium carbide MXene from a parent "MAX" ceramic containing titanium (denoted by "M"), aluminum ("A") and carbon ("X"). Ke Li did this by etching out the aluminum layers to form five-ply MXene monolayers of titanium carbide.

Afterward, the researchers soaked the MXenes in lithium-based electrolytes in various solvents with dramatically different molecular structures and properties. The lithium ions carried the electrical charge. Lithium ions easily insert themselves between MXene layers.

Transmission electron microscopy showed the structure of the materials before and after electrochemical experiments. They used X-ray photoelectron spectroscopy and Raman spectroscopy to characterize the MXene's composition and observe the interactions between the electrolyte solvent and MXene surface.

Electrochemical measurements revealed that the maximum capacitance (amount of energy stored) was achieved using a less conductive electrolyte. This observation was unusual and counterintuitive. One would expect a commonly used acetonitrile solvent-based electrolyte with the highest conductivity of all tested electrolytes, to provide the best performance.

Using in situ X-ray diffraction they observed expansion and contraction of the MXene interlayer spacing during charging and discharging when acetonitrile was used. No changes were seen in the interlayer spacing when the propylene carbonate solvent was used.

The solvent of a less conductive electrolyte resulted in much higher capacitance. Moreover, electrodes that don't expand when ions enter and exit are expected to survive a greater number of charge-discharge cycles.

Neutron scattering, which is sensitive to hydrogen atoms in the solvent, was used to probe the dynamics of the electrolyte solvent media confined in the MXene layers.

Finally, molecular dynamics simulations done by Vlcek showed that interactions among the lithium ions, electrolyte solvents, and MXene surfaces strongly depend on the polarity, molecular shape, and size of the solvent molecules.

For example, in a propylene carbonate-based electrolyte, the lithium ions are not surrounded by solvent and therefore can pack tightly between MXene sheets.

However, in other electrolytes, lithium ions can carry solvent molecules along with them as the lithium ions migrate into the electrode. This migration leads to its expansion upon charging.

Modeling may help guide the selection of future electrode-electrolyte solvent couples.

"Different solvents created different confined environments that then had profound influence on charge transport and interactions of ions with the MXene electrodes," Vlcek said. "This variety of structures and behaviors was made possible by the layered structure of MXene electrodes, which can respond to charging by easily expanding and contracting the interlayer space to accommodate a much wider range of solvents than electrodes with more rigid frameworks."

Details about the study and the findings were published in Nature Energy in a paper titled, "Influences from solvents on charge storage in titanium carbide MXenes."