Low Dimensional Composite Materials as Binder-Free Anode for Energy Storage Applications

The prolonged use of non-renewable energy such as fossil fuel and coal is having a deteriorating impact on our environment leading to severe climate changes. Hence, there is a dire need to explore new methods for energy generation and storage which are environmentally friendly, cost-effective and sustainable. Sodium-ion batteries have been attracting great interest as an alternative to lithium-ion batteries due to their material abundance, lower cost, and sustainability. However, a high-performance viable anode for the sodium-ion battery has yet to be discovered because of the difference in its molecular size and intercalation mechanism to the comparatively smaller lithium ions.<br/><br/>This issue has garnered global research interest in the quest to develop anodes with expanded interlayer spacing capable of accommodating sodium ions. This area of study has emerged as a promising field within material science, particularly in the realm of 2D materials research. 2D materials are known for their unique structure and electronic properties such as hardness, tuneable band gap by variation of the number of layers and their high surface area. These properties make them intriguing candidates for applications in batteries and electrocatalysis. Black phosphorus, a well-studied 2D class material, has previously been reported to show very high theoretical capacity for both sodium and lithium-ion battery systems but suffers from low cycling stability due to chemical and structural instability.^[1] To overcome this problem, we decided to explore the prospect of a new 2D composite material made from nanosheets of black phosphorus (phosphorene) and MXene (Ti₃C₂T_x) to be potentially used as an anode in a sodium-ion battery. The MXene layers with terminating fluorine and oxygen functionals are expected to promote the growth of stable solid-electrolyte interfaces (SEI) which in turn is expected to further improve the overall coulombic efficiency of the battery.^[2][3]<br/><br/>Both nanomaterials were synthesized via liquid-phase exfoliation (LPE). Phosphorene is notorious for being oxidized easily. Hence, to protect the phosphorene, a novel layer-by-layer (LBL) vacuum filtration technique was implemented to obtain the electrode. The resulting electrodes were then assembled in a lithium-ion coin cell to study their cycling stability and rate capacity, given the similarity of the lithium and sodium ion intercalation mechanism in the anode. XPS, SEM, EDX, UV-Vis, XRD and AFM characterization were performed on the composite nanomaterial to study its morphology, as well as compositional and structural changes upon processing. It was found that the electrode material showed a high discharge capacity of 700 mAh/g at 0.2 C with 98% coulombic efficiency for 100 cycles which indicates a promising performance potential for sodium ion battery systems.<br/><br/>In conclusion, the layer-by-layer MXene-phosphorene composite structure effectively shields the phosphorene nanosheets from extensive oxidation in ambient conditions. This preservation of phosphorene enables a reversible reaction and ensures excellent cycling stability as an electrode material. Moreover, the inclusion of the MXene component acts as a conductive binder in this composite eliminating the need for traditional non-conductive binders like PVDF and CMC. This substitution reduces dead volume within the anode and enhances the material's specific capacity. Finally, the composite's large interlayer spacing highlights its potential application as an anode in sodium ion battery systems, emphasizing the need for continued exploration and research in this area.<br/><br/>References<br/>Mohamed Alhabeb, et al., Chemistry of Materials, 2017, Vol. 29<br/>Junye Cheng et al., Nano-Micro Letters, 2020, Vol. 12.<br/>Xiong-Xiong Xue, et al., Journal of Physics: Energy, 2021, Vol. 3