An overview of hydroxy-based polyanionic cathode insertion materials for metal-ion batteries.

Rechargeable batteries based on Li-ion and post Li-ion chemistry have come a long way since their inception in the early 1980s. The last four decades have witnessed steady development and discovery of myriads of cathode materials taking into account their processing, economy, and performance along with ecological sustainability. Though oxides rule the battery sector with their high energy and power density, polyanionic insertion compounds work as gold mines for designing insertion compounds with rich structural diversity leading to tuneable redox potential coupled with high structural/chemical/thermal stability. The scope of polyanionic compounds can be taken a step further by combining two or more different types of polyanions to get suites of mixed polyanionic materials. While most cathodes are built with metal polyhedra constituted by oxygen (MOm|XOm, M = 3d metals, X = P, S, Si, B, W, etc., m = 3-6), in some cases, selected oxygen sites can form bonding with hydrogen to form OH/H2O ligands. It can lead to the family of hydroxy-based mixed-polyanionic cathode materials. The presence of hydroxy components can affect the crystal structure, local chemical bonding, and electronic, magnetic, diffusivity and electrochemical properties. Employing a mineralogical survey, the current review renders a sneak peek on various hydroxy-based polyanionic cathode materials for Li-ion and post Li-ion batteries. Their crystal structure, and electrochemical properties have been overviewed to outline future research focus and scope for real-life application.

Cobalt Metaphosphates as Economic Bifunctional Electrocatalysts for Hybrid Sodium-Air Batteries.

Bifunctional electrocatalysts are pre-eminent to achieve high capacity, cycling stability, and high Coulombic efficiency for rechargeable hybrid sodium-air batteries. The current work introduces metaphosphate (Na)KCo(PO3)3 nanostructures as noble metal-free bifunctional electrocatalysts suitable for the rechargeable aqueous sodium-air battery. Prepared by the scalable solution combustion method, the metaphosphate class of (Na)KCo(PO3)3 with spherical morphology exhibited robust oxygen reduction as well as evolution activity similar to the state-of-the-art catalysts. NaCo(PO3)3 metaphosphate, when employed as an air cathode in hybrid sodium-air batteries, delivered reasonably low overpotential along with excellent cycling stability with a round-trip energy efficiency of 78%. Cobalt metaphosphates thus form a new class of economical bifunctional catalysts to develop hybrid sodium-air batteries.