Na4Mn9O18 as a positive electrode material for an aqueous electrolyte sodium-ion energy storage device

Here we demonstrate Na4Mn9O18 as a sodium intercalation positive electrode material for an aqueous electrolyte energy storage device. A simple solid-state synthesis route was used to produce this material, which was then tested electrochemically in a 1 M Na2SO4 electrolyte against an activated carbon counter electrode using cyclic voltammetry and galvanostatic cycling. Optimized Na4Mn9O18 was documented as having a specific capacity of 45 mAh/g through a voltage range of 0.5 V, or an equivalent specific capacitance of over 300 F/g. With the proper negative:positive electrode mass ratio, energy storage cells capable of being charged to at least 1.7 V without significant water electrolysis are documented. Cycling data and rate studies indicate promising performance for this unexplored low-cost positive electrode material.