High purity Mn5O8 nanoparticles with a high overpotential to gas evolution reactions for high voltage aqueous sodium-ion electrochemical storage

Abstract

Developing electrodes with high specific energy by using inexpensive manganese oxides is of great importance for aqueous electrochemical energy storage (EES) using non-Li charge carriers such as Na-or K-ions. However, the energy density of aqueous EES devices is generally limited by their narrow thermodynamic potential window (~1.23 V). In this paper, the synthesis of high purity layered Mn5O8 nanoparticles through solid state thermal treatment of Mn3O4 spinel nanoparticles, resulting in a chemical formula of [Mn 2 2+ ] [Mn 3 4+ O82–], evidenced by Rietveld refinement of synchrotron- based X-ray diffraction, has been reported. The electro-kinetic analyses obtained from cyclic voltammetry measurements in half-cells have demonstrated that Mn5O8 electrode has a large overpotential (~ 0.6 V) towards gas evolution reactions, resulting in a stable potential window of 2.5 V in an aqueous electrolyte in half-cell measurements. Symmetric full-cells fabricated using Mn5O8 electrodes can be operated within a stable 3.0 V potential window for 5000 galvanostatic cycles, exhibiting a stable electrode capacity of about 103 mAh/g at a C-rate of 95 with nearly 100% coulombic efficiency and 96% energy efficiency.