Relating Electrolyte Concentration to Performance and Stability for NaTi2(PO4)3/Na0.44MnO2 Aqueous Sodium-Ion Batteries

Abstract

In aqueous electrolyte batteries, the salt concentration of the electrolyte affects the ionic conductivity, which will further affect the rate performance of the battery as well as the diffusion of reactive species that can cause self-discharge, particularly in thick electrode devices. To this end, we explored the implications of device performance using Na0.44MnO2 cathode material, NaTi2(PO4)3, anode material in a NaClO4-based aqueous solutions with molarities as high as 5. Experiments included physical property characterizations, cyclic voltammetry, and constant current charging/discharging methods. Preliminary results indicate that, in cells with thick electrodes (∼1mm), rate capability and electrode utilization increased significantly with higher molarity solutions: capacity at the 1.5 C rate increased 38% by increasing the salt concentration from 1 M to 5 M. At the same time the oxygen-related self-discharge phenomenon was diminished when using higher electrolyte molarities, though there was still measurable loss in capacity in in the electrodes. Irreversible capacity loss was observed to occur even in electrolytes with the lowest oxygen content, suggesting that self discharge and capacity loss are not necessarily causally related.