Cobalt Doping as a Pathway To Stabilize the Solid-State Conversion Chemistry of Manganese Oxide Anodes in Li-Ion Batteries

Abstract

Metal oxides have been widely studied in recent years to replace commercial graphite anodes in lithium ion batteries. Among the metal oxides, manganese oxide has a high theoretical capacity, low cost, and is environmentally friendly. However, many MnO materials have shown limited reaction reversibility and poor conversion kinetics. To understand why, in this paper we investigate the mechanism, kinetics, and reversibility for the solid-state conversion reaction of MnO with Li+. We definitively show, for the first time, that during repeated reaction cycles, multiple reaction pathways occur that lead not only to the reformation of MnO but also higher oxidation-state Mn3O4—which when combined with the poor intrinsic electronic conductivity of both manganese oxide species results in a rapid loss in the amount of charge that can be stored in these materials. Learning this, the approach in this study was to use cobalt doping to concomitantly stabilize the redox behavior of manganese (allowing for the gradual transformation of MnO to Mn3O4 over time) and to increase the intraparticle electronic conductivity of the active layer. The result is an active material, Mn0.9Co0.1O, that exhibits excellent charge stability and conversion kinetics (near 600 mAh/g at a rate of 400 mA/g), even over hundreds of reaction cycles.