Superior performance of calcium birnessite by electrochemical conversion as cathode for aqueous calcium ion battery

• Hierarchical Ca-birnessite with ultrathin and intertwined nanosheets structure converted from Mn 3 O 4 is successfully fabricated. • The hybrid Ca-birnessite cathode for aqueous calcium ion battery exhibt large capacity and good cycle performance. • A full cell of Ca-birnessite//PI is constructed and shows an excellent cycling stability with almost no capacity decay after 1500 cycles at 1 a g −1 . Aqueous Ca 2+ ion battery (ACIB) is known as a promising candidate for large-scale grid storage due to its low cost, high performance and safety. However, the large ion size of Ca 2+ (1.00 Å) limits the ability of Ca 2+ to (de)intercalate in the host structure. In this report, hierarchical Ca-birnessite with ultrathin and intertwined nanosheets structure converted from Mn 3 O 4 nanowall arrays (NWAs) was successfully fabricated via an electro-conversion method, showing a fast surface faraday reaction as well as favorable Ca 2+ storage kinetics and properties. When tested in ACIB, the electro-converted Ca-birnessite (ECCB) electrode exhibits an excellent specific capacity of 175 mAh g − 1 at 0.1 A g − 1 , high rate performance (89 mAh g − 1 at 2 A g − 1 ) and long cycling stability (83.2% retention after 2000 cycles at 1 A g − 1 ). A kinetic analysis indicates that the reaction mechanism is dominated by surface-limited capacitance. Moreover, we constructed a balanced 1.9 V full cell of ECCB//PI, which delivers a reversible capacity of 42.1 mAh g − 1 at 0.2 A g − 1 and an energy density of 30.56 Wh kg −1 at power density of 143.4 W kg −1 . Furthermore, the full cell shows an excellent cycling stability with almost no capacity decay after 1500 cycles at 1 A g − 1 . This work offers a new strategy for designing high performance cathodes for aqueous calcium ion battery.