Vacancy‐Driven High Rate Capabilities in Calcium‐Doped Na0.4MnO2 Cathodes for Aqueous Sodium‐Ion Batteries

Aqueous sodium‐ion batteries are expected to be low‐cost, safe, and environmentally friendly systems for large scale energy storage due to the abundance and low cost of sodium. However, only a few candidates have been reported for cathodes and there is a need to develop new practical host materials with improved electrochemical performance. Here, tunnel‐type, calcium‐doped, sodium manganese oxide is demonstrated as a novel cathode material, ultrafast rate capabilities and superior high‐rate cycling stability—98.8% capacity retention at the 1000th cycle—for aqueous sodium‐ion batteries. Advanced structural analysis of the Ca0.07Na0.26MnO2 material using X‐ray diffraction and ab initio calculations identify the calcium sites and indicate a plausible sodium diffusion mechanism. Calcium preferentially substitutes at the Na(1) sites among the three different types of Na sites. This substitution creates vacancies at the Na(2) and Na(3) sites. Calculations of the energy barrier for Na ion diffusion indicate that diffusion along the Na(2)‐to‐Na(2) and Na(2)‐to‐Na(3) pathways is the most feasible. These vacancies provide improved diffusion kinetics and show 43% capacity enhancement at 50 C‐rate. The results suggest that Ca0.07Na0.26MnO2 is a promising cathode material for aqueous sodium‐ion batteries, and provide an improved fundamental understanding of sodium storage mechanisms. Calcium doping boosts Na0.4MnO2 electrochemical performance via enhanced diffusion kinetics. The intricate sodium storage mechanism is revealed by combined electrochemical characterization, structure determination by powder X‐ray diffraction, and energy‐barrier calculations of sodium diffusion.