Vacancy-driven high rate capabilities in calcium-doped Na_0.4MnO₂ cathode for aqueous sodium-ion batteries

The aqueous sodium-ion batteries are low-cost, safe, and environmental friendliness system for large scale energy storage technologies, because of the abundance and inexpensive of sodium source. Recently, few cathode candidates are reported, but still need to develop new host material to improve their electrochemical performances. Herein we demonstrate tunnel-type calcium doped sodium manganese oxide, Ca_0.07Na_0.26MnO₂, which shows ultrafast rate capabilities and superior high-rate cycling stability (98.8% capacity retention at the 1000^th cycle) for aqueous sodium-ion batteries. Advanced structure analyses using X-ray diffraction and ab initio calculations are utilized to accurately identify the calcium sites and sodium storage mechanism. Calcium substitution offer the sodium vacancy site in Na(2) and Na(3) site. This vacancy makes improving the materials diffusion kinetics and it shows 43 % of capacity improvement at 50 C-rate. Our results clearly demonstrate the fundamental understanding and practical demonstrations suggest that Ca_0.07Na_0.26MnO₂ is a promising cathode material for aqueous sodium-ion batteries.