A team of researchers at the National University of Singapore (NUS) have developed an ultra-thin, two-dimensional polymer sheet, the organic analogue of graphene, which is electrically conductive and has regular, sub-nanometer pores that can be used to store sodium ions safely in sodium-ion batteries.
NUS PhD student Mr Liu Wei showing the two-dimensional graphene-like polymer (left), and a piece of carbon
paper coated with the novel material (right). (NUS)
According to a report from the university, this 2-D polymer can be mass-produced at low cost and can be used as the electrode for sodium-ion batteries at high capacity over thousands of charge cycles. This is a huge breakthrough for sodium-ion batteries, which had been limited by short lifespans.
The scientists discovered the means for linking 2-D polymers with carbon-carbon linkages that allow the polymer to withstand harsh environments, such as being submerged in water or acids or being heated. A planar molecule with several aromatic rings was used by the scientists. The rings consisted of connected carbon double bonds that enabled the movement of electrons.
The article explained, “During their experiments, the NUS researchers discovered that by applying heat to the set of carefully designed, flat monomers which are pre-packed in a specific way, a 2D crystalline polymer is formed. The 2D polymer consists of well-defined pores and channels, through which sodium ions can diffuse in and out for energy storage.”
The material demonstrated stability and conductivity, which sets it apart from other organic compounds attempted for sodium-ion battery use. As the battery’s anode, the material could be charged and discharged at room temperature and kept 70% of its capacity after 7,700 charge cycles.
The research was recently published in Nature Chemistry. The abstract of the report stated:
“The fabrication of crystalline 2D conjugated polymers with well-defined repeating units and in-built porosity presents a significant challenge to synthetic chemists. Yet they present an appealing target because of their desirable physical and electronic properties.
“Here we report the preparation of a 2D conjugated aromatic polymer synthesized via C–C coupling reactions between tetrabromopolyaromatic monomers. Pre-arranged monomers in the bulk crystal undergo C–C coupling driven by endogenous solid-state polymerization to produce a crystalline polymer, which can be mechanically exfoliated into micrometre-sized lamellar sheets with a thickness of 1 nm.
“Isothermal gas-sorption measurements of the bulk material reveal a dominant pore size of ~0.6 nm, which indicates uniform open channels from the eclipsed stacking of the sheets.
“When employed as an organic anode in an ambient-temperature sodium cell, the material allows a fast charge/discharge of sodium ions, with impressive reversible capacity, rate capability and stability metrics.”