Cobalt doped K-birnessite as ultrastable cathode for aqueous calcium-ion batteries

Aqueous Ca ion batteries (ACIBs) are one of the emerging energy-storage technologies due to their low costs and low polarization strength. Birnessite-MnO2 is one kind of promising cathode because of its large interlayer distance for facile Ca2+ ion intercalation and high theoretical specific capacity (241 mAh g−1), but the inferior cycling performance significantly limits its application in ACIBs. Herein, 2% Co-doped K-birnessite K0.11Co0.02Mn0.98O2·1.4H2O (CB-2) with superior structural stability is reported for the first time. The Co-doped K-birnessite exhibits enhanced conductivity and capacitance ability. Moreover, after Co doping, the Mn–O ionic bond is shortened and the banding energy is reduced, which effectively inhibits the Jahn-Taller effect and improves the thermodynamic stability. More interestingly, we find that Ca2+ can restrain the irreversible transition of birnessite from layered to spinel. CB-2 cathode delivers an excellent rate performance of 181.2 mAh g−1 at 200 mA g−1 and structural stability of 1000 cycles at 2 A g−1 with 90.4% capacity retention. Finally, a stable ACIB is constructed by using the CB-2 cathode and polyimide anode, which shows a decent specific capacity of 51.9 mAh g−1 at 200 mA g−1 and superior cycling stability. This demonstrates effective foreign metal ion doping for K-birnessite to achieve superior Ca2+ storage in aqueous batteries.