Treating MXene electrodes with laser to improve Li-ion battery performance

Problems with using MXenes

Lithium-ion batteries use graphite electrodes which contain layers of carbon atoms. When a battery charges, lithium ions are stored between these layers in a process that scientists refer to as intercalation.

MXenes are preferred over graphite as electrode material since they provide additional storage space for lithium ions to intercalate. The problem, however, is that higher storage capacity diminishes after repeated charge and discharge cycles.

Researchers at KAUST found that the cause of the capacity degradation was a chemical change that led to molybdenum oxide formation within the MXene structure.

Boosting performance with lasers

The research team led by Husam N. Alshareef used a process called laser scribing, where pulses of infrared lasers were used to create "nanodots" on molybdenum carbide on the MXene electrodes. These nanodots were approximately 10 nanometers wide and were connected to the MXene layers with carbon materials, the press release said.

The laser-scribed material was used to make an anode and tested in a lithium-ion battery over 1,000 charge-discharge cycles. The researchers found that the anode with nanodots had four times higher electrical storage capacity than the one without and was also capable of reaching the theoretical maximum capacity of graphite. Moreover, there was no drop in performance even after 1,000 cycles.

The researchers attribute the improved performance of the laser-scribed material to multiple factors. The creation of nanodots provides additional storage space for lithium ions to intercalate, speeding up the charging process. It also reduces the oxygen content in the material, further preventing the formation of molybdenum oxide and degrading MXene electrode performance.

The connections between the nanodots and the layers further improve the material's conductivity and stabilize its structure. The researchers are confident that the approach could be applied as a strategy to improve the performance of MXenes that also use other metals.

While lithium prices are through the roof these days due to their high demand, MXenes can also work with more abundant metal ions such as sodium and potassium. This could also lead to the development of a new generation of rechargeable batteries.

"This provides a cost-effective and fast way to tune battery performance," added Zahra Bayhan, who worked on the approach as a Ph.D. student at KAUST.

The research findings have been published in the journal Small.

Abstract

MXenes, a fast-growing family of two-dimensional (2D) transition metal carbides/nitrides, are promising for electronics and energy storage applications. Mo2CTx MXene, in particular, has demonstrated a higher capacity than other MXenes as an anode for Li-ion batteries. Yet, such enhanced capacity is accompanied by slow kinetics and poor cycling stability. Herein, it is revealed that the unstable cycling performance of Mo2CTx is attributed to the partial oxidation into MoOx with structural degradation. A laser-induced Mo2CTx/Mo2C (LS-Mo2CTx) hybrid anode has been developed, of which the Mo2C nanodots boost redox kinetics, and the laser-reduced oxygen content prevents the structural degradation caused by oxidation. Meanwhile, the strong connections between the laser-induced Mo2C nanodots and Mo2CTx nanosheets enhance conductivity and stabilize the structure during charge–discharge cycling. The as-prepared LS-Mo2CTx anode exhibits an enhanced capacity of 340 mAh g−1 vs 83 mAh g−1 (for pristine) and an improved cycling stability (capacity retention of 106.2% vs 80.6% for pristine) over 1000 cycles. The laser-induced synthesis approach underlines the potential of MXene-based hybrid materials for high-performance energy storage applications.