Polyanion Compounds As Positive Electrode Materials for Rechargeable Aqueous Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are potential next-generation batteries that can alleviate the cost and resource challenges of lithium. Developing aqueous SIBs is more desirable to emphasize cost-effective factor combining with abundance of Na resources. Aqueous electrolytes exhibit higher ionic mobility as well as are cheaper than organic solvents. Moreover, safety concerns using flammable organic electrolytes could be resolve, therefore aqueous SIB systems would be safer and cheaper than the current organic solvent based LIBs. However, use of aqueous electrolytes imposes a significant challenge in finding appropriate electrode materials that operate within the limited potential window of water while generating reasonably high cell-voltages. Here we introduce two polyanion compounds with carbonaceous composites as promising electrode materials for aqueous SIBs. As the 3-dimensional frameworks of polyanion compounds can accommodate large Na ions and provide easy diffusion path as well as the structural stability, these materials show good cyclability and rate performances especially in safe and environmentally benign aqueous systems. High stability and excellent electrochemical properties of two polyanion compounds, Na₂FeP₂O₇ and Na₃V₂O_2x(PO₄)₂F_3-2x, in aqueous electrolytes will be introduced.