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

Aqueous Ca²⁺ 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 Ca²⁺ (1.00 Å) limits the ability of Ca²⁺ to (de)intercalate in the host structure. In this report, hierarchical Ca-birnessite with ultrathin and intertwined nanosheets structure converted from Mn₃O₄ nanowall arrays (NWAs) was successfully fabricated via an electro-conversion method, showing a fast surface faraday reaction as well as favorable Ca²⁺ 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.