The development of more efficient, sustainable and cost-effective energy storage devices is a bottleneck in the deployment of intermittent renewable energies. Furthermore, new energy storage formats, where a high energy storage is supplied in small volumes, are required to serve a range of new applications including wearable electronics.

Supercapacitors are energy storage devices that offer a high power density, as compared to batteries, but suffer from a comparatively poorer energy density, a main drawback of these devices. In this work, we achieved a significant enhancement of the energy per unit volume that can be stored in a supercapacitor half-cell device consisting of a V₄C₃T_Z MXene and a highly concentrated electrolyte.

MXenes are two-dimensional materials that perform at high rates and are particularly suitable to power wearable electronics due to their mechanical flexibility. However, a problem that most to date reported MXene-aqueous electrolyte systems face is a limited operation electrochemical window, which under basic principles, determines its energy density. The problem lies on common undesired water-decomposition electrochemical reactions occurring in standard (low concentration of 1 molar) aqueous electrolytes at onset potentials defined mostly by the nature of electrode-electrolyte interfaces. In the case of MXenes, a common electrochemical window in standard aqueous electrolytes is 0.55 V.

In this work, we have formulated a highly concentrated (17.5 molal) electrolyte based on LiBr salt and very few water molecules, in which an entirely new molecular structure induces new electrochemical and physicochemical processes that “delay” the onset potential of undesired water-decomposition reactions. The resulting enhanced electrochemical window is 1.5 V, that is 3-fold, the electrochemical window in standard aqueous electrolytes. This resulted in a 49.4 Wh kg^-1/155.3 mWh cm^-3 energy per unit mass and volume, respectively. A process contributing to the enhancement of the electrochemical window was a novel electrode-electrolyte interfacial film elucidated using electrochemical impedance spectroscopy and enabled by the highly concentrated electrolyte.

A great part of the success of this half-cell device is the high quality of the achieved 2D V₄C₃T_Z MXene, i.e., one and few layers material, which enabled a high surface area, that in turn led to a high energy density, which combined with a high electrical conductivity, mechanical integrity and stability, and the presence of vacancies – that favored ion transport – led to a high power density and a cycling stability up to 10,000 cycles.

The work entitled “A high capacity V₄C₃T_Z MXene electrode: expanding the limits of stable electrochemical windows using a highly concentrated LiBr/H₂O electrolyte” was published in Energy Materials (published on Aug. 28, 2025).
DOI: 10.20517/energymater.2024.280