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.