Identifying the Active Species in Li-Na Dual-Ion "Saltwater Battery" Based on Spinel Lithium Manganese Oxide, Sodium Titanium Phosphate and Aqueous Electrolyte

The dual-ion "Saltwater Battery" based on aqueous electrolyte containing sodium ions and lithium ions is believed to be one of the safest and environmentally friendliest battery technologies. The anode consists of sodium titanium phosphate, whereas the cathode is spinel lithium manganese oxide. It has been reported that both materials can intercalate sodium and lithium ions depending on their availability in the electrolyte. This study aims to identify the dominant active species in "Saltwater Batteries" with dual-ion electrolyte. Therefore, cyclic voltammetry of single electrodes as well as full cell measurements are performed with electrolyte containing lithium sulfate, sodium sulfate or a mixture. Moreover, the study is complemented by ion analysis of the electrolyte, as well as X-ray diffraction of fresh and cycled electrodes at different states of charge. The results show that the cathode only (de)intercalates lithium ions, revealing that Li is the dominant active cathode species. The anode can react with both ions and undergoes a formation reaction accompanied by partial dissolution of sodium titanium phosphate. Nevertheless, cyclic voltammetry and full cell measurements indicate that lithium is also the dominant active species on the anode side. In conclusion, the dual-ion battery is dominated by lithium and shows a superior performance when removing sodium from electrolyte.