An electric double layer capacitor that uses a phenomenon called "electric double layer" in which electrons and ions are paired is a kind of power storage device, and has almost no deterioration due to repeated use, and has a higher output than an ion battery.In order to store electricity efficiently, it is necessary to confine electrons and ions at high density in a minute space (nano space).At this time, water molecules bound to ions are also trapped, but since the characteristics of these water molecules are unknown, we did not know how to effectively trap electrons and ions.

　The research group created an electric double layer capacitor using the layered compound MXene as an electrode material and used various alkali metal ions, measured the electric double layer capacitance, and carried out a calculation simulation.Analysis revealed that water molecules confined with lithium ions in the MXene interlayer nanospace have a negative permittivity, that is, they form an abnormal potential distribution in the nanospace that produces an internal electric field in the direction opposite to the external electric field. bottom.Furthermore, it was demonstrated from both experimental and theoretical perspectives that the negative permittivity of water increases the electric double layer capacity of the layered compound by 1.7 times compared to the case of using ions that do not show a negative permittivity.

　In addition, the negative permittivity was confirmed by experiments and theory in other layered compounds such as graphene composed only of carbon, and it was found to be a universal phenomenon.By using the negative permittivity, it is possible to store ions at high density with less energy, which is expected to lead to the development of electric double layer capacitors with high energy density.

Paper information:[Nature Communications] Negative permittivity of water confined in nanosheets