Investigation on AxMn3O7 Class of Layered Oxides as Versatile Battery Cathodes: Structure and Electrochemistry

Abstract

Rechargeable battery technologies have been the spotlight of global scientific research in the quest to address steadily growing energy demand. Currently, Li-ion batteries have unanimously occupied consumer electronic sector. Most of these batteries are majorly based on two transition elements, Cobalt (Co) and Nickel (Ni), in popular cathodes like LiCoO2 and LiNi1/3Mn1/3Co1/3O2 (NMC). The resource constraints of Co and Ni have paved away for alternate transition metal redox chemistry. In this scenario, manganese based layered materials offer promise owing to their low-cost, resource-friendly nature with high operational safety. The cathode (positive electrode) forms the central component accounting for over 35% of the net cost of battery. The current work revolves around the rational design of high energy density cathode materials synergizing “Synthesis-Structure-Electrochemistry-Mechanism” of economic A2Mn3O7 (A= Li, Na, K, and Zn) class of insertion materials. Triclinic Na2Mn3O7 (P-1, #2) consists of defective (vacant sites) infinite [Mn3O7]-2¥ layers with two distinct Na-sites. The flexibility of variable Mn oxidation states triggers polymorphism and rich crystal chemistry. The Mn+4 oxidation state specifies the ability of layered Na2Mn3O7 to accommodate foreign intercalating alkali-ions with active Mn+4/Mn+3/Mn+2 redox couples. Owing to the low bond valence site energy (BVSE) of the two Na-sites (0.27 eV) in Na2Mn3O7, they can be easily exchanged using simple soft-chemistry techniques. This work demonstrates Na2Mn3O7 as a versatile cathode for secondary Li-ion and post-Li-ion batteries