Two-dimensional MXene has been a rising star in the energy world because of its ability to store energy quickly. But its unstable voltage output limits its applications. A collaborative research team led by scientists from City University of Hong Kong (CityU) has recently developed battery-like electrochemical Nb2CTx MXene electrodes with stable voltage output and high energy density by using a high-voltage scanning strategy. These latest findings may lead to a breakthrough in inventing a next-generation powerful battery.

MXene is a large family of two-dimensional nanomaterials that has been the research focus of 2D materials in the energy storage field for the past decade. Because of its excellent electronic conductivity and large surface area, MXene features fast surficial redox and demonstrates high- rate energy storage.

But the biggest challenge of MXene energy storage is that all reported MXene electrodes lack a distinct discharge voltage plateau, which means they discharge with a rapidly descending output voltage. This shortcoming deteriorates Mxene’s energy density and stable energy output at desired high voltage regions, leading to limited energy density, usually less than 100 Wh/kg.

To overcome the problem of unstable energy output, the research team led by CityU successfully developed battery-type Nb2CTx MXene electrodes. The team revealed the completely different electrochemical properties of the Nb2CTx MXene electrode by regulating the voltage windows from 2.0V to 2.4V. Under high-voltage scanning up to 2.4V, the Nb2CTx MXene electrode showed typical battery-type features, different from the one under low voltage and other previously reported MXene systems.

The team discovered that the Nb2CTx/Zn battery could exhibit superior rate capability, durable cyclic performance, and high energy density under high-voltage scanning. More importantly, they succeeded in equipping MXene with a flat and stable discharge plateau of 1.55V to boost energy density. A record-level energy density among all aqueous Mxene electrodes, of 146.7 Wh/kg, with 63% contribution from the plateau region was also obtained. It broke the performance bottleneck of MXene devices.

“The absence of distinct voltage plateaus deteriorates MXene electrodes’ capacities and energy densities, which limit their potential as high-performance batteries. Our work successfully outlines an efficient route toward achieving high-energy-density MXene electrodes with distinct discharge voltage plateaus through a high-voltage-scanning approach, which dramatically improves their electrochemical performance,” said Professor Zhi Chunyi.

Professor Zhi believes that the findings will inspire more researchers to explore the unrevealed electrochemical properties of the MXene family. “Two-dimensional MXene, featured by fast surficial redox and high-rate energy storage, have outstanding energy storage performance. With the stable voltage output and greatly enhanced energy density, MXene-based energy storage devices are one step closer to the goal of practical application,” he said.

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