Layered Sodium Manganese Oxide Na2Mn3O7 as an Insertion Host for Aqueous Zinc-ion Batteries

Aqueous rechargeable batteries are attractive owing to their higher operational safety, high ionic conductivity, scalable and easy manufacturing. These aqueous batteries form an economic option for large-scale (grid) power storage. In the aqueous battery sector, Mn-based compounds are highly attractive with their non-toxic nature, low-cost, rich mineral chemistry and robust operational safety. Several Mn-based systems like LiMn2O4 spinel and LiNi1/3Mn1/3Co1/3O2 have seen successful commercialization. Pursuing Mn-based materials, we have shown layer structured Na2Mn3O7 as a versatile cathode material for non-aqueous systems like Li-, Na- and K-ion batteries. In the current work, we have exploited Na2Mn3O7 as a cathode material for aqueous Zn-ion battery for the first time. This Na-Mn-O ternary system was prepared using two-step emulsion-based synthesis. The phase-pure Na2Mn3O7 was formed in a triclinic structure with a space group of P-1. It exhibited versatile electrochemical insertion of different ions like Li-, Na- and K-ions involving phase transition. Na2Mn3O7 exhibited reversible Zn-ion intercalation delivering capacity of 245 mA h g(-1) with a nominal voltage of 1.5 V. Upon discharge, it triggered phase transformation to an unknown phase. Layered Na2Mn3O7 oxide was found to act as an efficient cathode for Zn-ion batteries with good cycling stability.