Aqueous calcium-ion storage in amorphous molybdenum oxide

Aqueous Ca-ion batteries (ACIBs) show great potential for grid-scale energy storage applications due to their low cost, high safety, and environmental sustainability. However, their development is limited by the lack of proper Ca2+-storage materials. Herein, the electrochemical Ca2+ storage chemistry of an electrodeposited molybdenum oxide (MoOx) material is studied for the first time. The d-electron concentration of Mo and the content of structural water in MoOx play important roles in Ca2+ storage. The MoOH components in MoOx could suppress material dissolution in the weakly alkaline CaCl2 electrolyte. As expected, the MoOx electrode delivers a good discharge capacity of 106 mAh g−1 at 0.1 A g−1 (mass loading: 10 mg cm−2), and ultra-long cycle life of 30 000 cycles at 3.0 A g−1, outperforming the state-of-the-art Ca2+ host materials. In depth mechanism study illustrates that MoOx experiences a pure Ca2+ insertion/deinsertion mechanism during charge/discharge processes. Finally, an aqueous Ca-ion full cell is assembled using MoOx anode and copper hexacyanoferrate (CuHCF) cathode, exhibiting a high areal energy density of 0.96 mWh cm−2 (42.7 Wh kg−1) and long cycle life. Importantly, the Ca2+ storage in MoOx points to a new paradigm of Ca2+-based aqueous energy storage.