Accessing the second electron capacity of MnO2 by exploring complexation and intercalation reactions in energy dense alkaline batteries

MnO2 is one of the safest and most abundant electrochemical materials available. It exhibits polymorphism, which has been exploited in many applications especially batteries. In alkaline batteries, y-MnO2 is widely used because its proton insertion reaction yields one e(-) (1st) per Mn at high voltages. It also gives a second (2nd) e(-) during dissolution precipitation reactions that occur at lower voltages than the proton insertion; however, these 2nd e(-) reactions are highly irreversible. In this communication, we explore the reversibility of the 2nd e(-) reactions with bismuth oxide (Bi2O3) and copper (Cu) additives, and cycling in specific potential regions where 8-MnO2 is the polymorph synthesized electrochemically in-situ from y-MnO2. The use of Bi2O3 and Cu add complexation and intercalation reactions, where presence and/or electrochemical activation of both are essential for reversibility and for capacity retention (50-100%). Attaining 300-610Wh kg(-1) against a zinc anode is possible for these batteries, which could promote its use for many applications. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.