Novel High-Energy-Density Rechargeable Hybrid Sodium–Air Cell with Acidic Electrolyte

Low-cost, high-energy-density, and highly efficient devices for energy storage have long been desired in our society. Herein, a novel high-energy-density hybrid sodium–air cell was fabricated successfully on the basis of acidic catholytes. Such a hybrid sodium–air cell possess a high theoretical voltage of 3.94 V, capacity of 1121 mAh g–1, and energy density of 4418 Wh kg–1. First, the buffering effect of an acidic solution was demonstrated, which provides relatively long and stable cell discharge behaviors. Second, the catholytes of hybrid sodium–air cells were optimized systematically from the solutions of 0.1 M H3PO4 + 0.1 M Na2SO4 to 0.1 M HAc + 0.1 M NaAc and it was found that the cells with 0.1 M H3PO4 + 0.1 M Na2SO4 displayed a maximum power density of 34.9 mW cm–2. The cell with 0.1 M H3PO4 + 0.1 M Na2SO4 displayed higher discharge capacity of 896 mAh g–1. Moreover, the fabricated acidic hybrid sodium–air cells exhibited stable cycling performance in ambient air and they delivered a low voltage gap around 0.3 V when the current density is 0.13 mA cm–2, leading to a high energy efficiency up to 90%. Therefore, the present study provides new opportunities to develop highly cost-effective energy storage technologies.