A stable anthraquinone-derivative cathode to develop sodium metal batteries: The role of ammoniates as electrolytes

Rechargeable sodium metal batteries constitute a cost-effective option for energy storage although sodium shows some drawbacks in terms of reactivity with organic solvents and dendritic growth. Here we demonstrate that an organic dye, indanthrone blue, behaves as an efficient cathode material for the development of secondary sodium metal batteries when combined with novel inorganic electrolytes. These electrolytes are ammonia solvates, known as liquid ammoniates, which can be formulated as NaI·3.3NH3 and NaBF4·2.5NH3. They impart excellent stability to sodium metal, and they favor sodium non-dendritic growth linked to their exceedingly high sodium ion concentration. This advantage is complemented by a high specific conductivity. The battery described here can last hundreds of cycles at 10 C while keeping a Coulombic efficiency of 99% from the first cycle. Because of the high capacity of the cathode and the superior physicochemical properties of the electrolytes, the battery can reach a specific energy value as high as 210 W h kg−1IB, and a high specific power of 2.2 kW kg−1IB, even at below room temperature (4 °C). Importantly, the battery is based on abundant and cost-effective materials, bearing promise for its application in large-scale energy storage.