Oxygen Evolution Reaction Enables Better Performance for Aqueous Na-Ion Batteries

Abstract

Aqueous batteries have attracted intensive attention owing to their inflammable water-based electrolyte. However, the upper cut-off voltage is significantly limited by the oxygen evolution reaction (OER) induced by water decomposition, which undermines the energy density of most aqueous batteries. OER not only consumes electrolytes but also leads to severe side reactions between generated oxygen and electrodes, resulting in rapid capacity decay. One of the most important aspects in maintaining a long aqueous battery lifetime is to suppress oxygen gas generation or eliminate oxygen gas in the cell. Although the negative effects of OER are well accepted, the positive influence is still ambiguous. In this presentation, we will discuss how properly manipulating OER can serve as an approach to improving the performance of aqueous batteries.Na-deficient Prussian Blue (PB) cathodes are undesired for aqueous full-cells due to the insufficient amount of cyclable Na-ions and inferior specific capacity. Our study will demonstrate that the performance of Na-deficient PB cathodes based full cells can be improved by OER. Relying on the two-electrode glassware cell composed of Na-poor PB cathode and NaTi2(PO4)3 (NTP) anode, the bubble generation on the cathode was directly observed during overcharging process, confirming the occurrence of OER at high charging states. After the overcharging protocol was applied to the coin cell during the first cycle, the discharge capacity was elevated from 55 mAh/g, which originates from low Na content in the pristine cathodes, to 130 mAh/g. Further analysis on structural evolution and charge compensation mechanisms revealed that, accompanying the OER on the cathode, Na ions in the electrolyte get inserted into the NTP anode during overcharging process, resulting in more cyclable Na-ions in the full cell system and thus enhancing the discharge capacity. On the other hand, owing to the pH gradient on the cathode surface caused by water decomposition, transition metal dissolution was reported through X-ray fluorescence mapping, leading to the declined Coulombic efficiency. Our study illustrates the potential utilization of overcharging protocol and offers fundamental insights into the influence of oxygen evolution reaction in aqueous batteries.