Low Dimensional MXene Composites for Next-Generation Energy Solutions

Abstract

The continued dependence on non-renewable energy sources, such as fossil fuels, has led to severe environmental consequences, driving the search for sustainable energy solutions. Among these, lithium-ion batteries (LIBs) have emerged as the leading energy storage technology due to their high energy density and efficiency. However, the increasing energy demands of modern applications necessitate improvement to LIBs, especially through the development of high-performance anode materials to replace graphite. At the same time, the growing demand for lithium and concerns over its resource availability have sparked interest in sodium-ion batteries (SIBs). SIBs offer advantages in terms of cost and resource abundance, but face significant challenges in anode fabrication due to the sodium's larger ionic radius, which leads to substantial volumetric expansion during cycling. Two-dimensional (2D) materials with expanded interlayer spacing offer a promising solution, particularly for alloying and conversion anodes such as silicon, tin, and black phosphorus. Their tunable electronic properties, high surface area, and mechanical strength make them attractive for mitigating volume change and improving electrochemical performance.

This thesis examines two promising 2D layered materials - phosphorene and tin (II) selenide (SnSe) - as active materials in anodes for metal-ion batteries. Both black phosphorus and SnSe demonstrate high theoretical capacities for sodium and lithium-ion storage; however, their practical application is hindered by low cycling stability due to chemical instability and structural degradation over repeated cycles. Ti3C2Tx MXene was integrated into each of the materials to form a composite, which facilitated a conductive network, enhancing electron transfer and reinforcing structural integrity. This design also eliminated the need for traditional binders, reducing dead volume within the electrode and increasing the gravimetric capacity. Furthermore, MXene's layered structure was demonstrated to accommodate volume changes, making it an effective conductive matrix for both phosphorene and SnSe-based composites.

To address the oxidation sensitivity of phosphorene, we employed a novel layer-by-layer vacuum filtration technique to create a phosphorene-MXene composite. This approach was demonstrated to mitigate the oxidative degradation of the phosphorene, enabling reversible reactions and enhanced cycling stability. The phosphorene-MXene composite achieved a high capacity of 700 mAh/g when cycled at 0.2 C for 100 cycles in lithium-ion half-cell testing. Despite this promising performance, the composite exhibited a low initial coulombic efficiency and poor reproducibility.

The SnSe-MXene composite, on the other hand, demonstrated an impressive capacity of 1148 mAh/g over 150 cycles and 900 mAh/g at 200 cycles, at charge-discharge rates of 0.2 C and 0.5 C, respectively, for lithium-ion half cells. The cycling data showcased an increasing capacity trend, attributed to the electro-milling effect of the SnSe nanoparticles combined with the participation of MXene in the electrochemistry. These findings underscore the composite's stability and capacity retention, with MXene's conductive network enhancing the performance of the composites.

The optimized SnSe-MXene composite was evaluated in sodium-ion half-cells using three different electrolyte solvents to assess their impact on cycling stability, capacity retention, and reaction reversibility. Among the tested electrolytes, diglyme exhibited the best performance, delivering a capacity of 580 mAh/g at 0.2 C after 160 cycles and 374 mAh/g at 0.5 C after 300 cycles.

In conclusion, our findings indicate that MXene-based composites, when combined with alloying and conversion materials such as phosphorene and SnSe, offer promising pathways for achieving stable, high-capacity anodes. The layered structure and conductive properties of MXene contributed to its effectiveness as a conductive binder, enhancing both the mechanical stability and electrochemical performance of the composites. These results highlight the potential of MXene-based composites in addressing the limitations of current anode materials, paving the way for further innovations in sustainable, high-performance metal-ion batteries.