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

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 (similar to 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+) O-8(2-)], 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 (similar to 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.