Aqueous Zn-MnO2 battery: Approaching the energy storage limit with deep Zn2+ pre-intercalation and revealing the ions insertion/extraction mechanisms

Rechargeable aqueous zinc ion batteries (AZIBs) were considered as one of the most promising candidates for large-scale energy storage due to the merits of high safety and inexpensiveness. As AZIBs cathode material, MnO2 possesses great merits but was greatly hindered due to the sluggish diffusion kinetic of Zn2+ during electrochemical operations. Herein, deep Zn2+ ions intercalated δ-MnO2 (Zn-MnO2) was achieved by the in situ electrochemical deposition route, which significantly enhanced the diffusion ability of Zn2+ due to the synergistic effects of Zn2+ pillars and structural H2O. The resultant Zn-MnO2 based AZIBs delivers a record capacity of 696 mAh/g (0.5 mAh/cm2) based on the initial mass loading, which is approaching the theoretical capacity of MnO2 with a two-electrons reaction. In-situ Raman studies reveal highly reversible Zn2+ ions insertion/extraction behaviors and here the Zn-MnO2 plays the role of a container during the charge–discharge process. Further charge storage mechanism investigations point out the insertion/extraction of Zn2+ and H+ coincides, and such process is significantly facilitated results from superior interlayered configurations of Zn-MnO2. The excellent electrochemical performance of Zn-MnO2 achieved in this work suggests the deep ions pre-intercalation strategy may aid in the future development of advanced cathodes for AZIBs.