Cation/anion synergistic modulation in birnessite-type MnO₂ to boost dual-ion storage in a wide temperature range

Aqueous Zn-ion batteries with high theoretical capacity, low price and high safety are promising energy storage devices. However, their development is restricted by manganese oxide (MnO₂) cathodes, which have the drawbacks of inferior electron conduction, irreversible dissolution and limited stability. Herein, a synergistic cation/anion (Ca/N) co-doping strategy is introduced to modulate the electric structure of birnessite-type MnO₂ (CACO-δ-MnO₂) to enhance its electron/ion-migration capability, improve structural stability and promote electrochemical performance. Theoretical analysis reveals that the synergistic effects of Ca/N co-doping considerably reduces the bandgap, decreases the adsorption energy and lowers the ion-diffusion energy barrier for CACO-δ-MnO₂, thereby increasing its conductivity and ion-diffusion capability. Ex situ and in situ analyses reveal that the dual ions of the Zn²⁺ and H⁺ insertion mechanism and the superior structural stability of CACO-δ-MnO₂ during charging/discharging processes. Galvanostatic intermittent titration technique and electrochemical impedance spectroscopy results confirm its fast kinetics, low charge-transfer resistance and good stability. Moreover, electrochemical tests demonstrate its superior stability in high-rate and long-term cycling, which delivers a high capacity of 213.5 mAh g^-1 following 2000 cycles at a high current density of 4 A g^-1. Integrated with the CACO-δ-MnO₂ cathode, the flexible Zn-MnO₂ battery demonstrates high flexibility and reliability in a wide temperature range (-20 °C-25 °C). Moreover, a long lifespan of 1000 cycles and a high capacity retention of 89.2 % are obtained for the flexible Zn-MnO₂ battery even at a low temperature of -20 °C. Therefore, this study not only introduces a novel Ca/N synergistic modulation strategy for high-performance δ-MnO₂ cathode but also propels the development of advanced Zn-ion batteries for various electronic items.