Rechargeable aqueous Zn-ion batteries (ZIBs) are very promising for large-scale grid energy storage applications owing to their low cost, environmentally benign constituent elements, excellent safety, and relatively high-energy density. Their usage, however, is largely hampered by their fast capacity fade. The cycle stability seems to be highly rate-dependent, which poses an additional challenge, but can also play a pivotal role in uncovering the reaction mechanisms. The complexity of the reactions in this electrochmical system has resulted in long-standing ambiguity of the chemical pathways of Zn/MnO₂ rechargable battery system, and has led to many controversies with regard to their nature. In this talk, we present a combined experimental and theoretical study of Zn/MnO₂ cells. We found that both H⁺/Zn²⁺ intercalation and conversion reactions occur at different voltages, and that the rapid capacity fading can clearly be ascribed to the rate-limiting and irreversible conversion reactions at a lower voltage. By avoiding the irreversible conversion reactions at ~ 1.26 V, we successfully demonstrate ultra-high power and long-life Zn/MnO₂ cells which, after 1000 cycles, maintain an energy density of ~ 231 Wh kg^-1 and 105 Wh kg^-1 at a power density of ~ 4 kW kg^-1 (9C, ~ 3.1 A g^-1) and ~ 15 kW kg^-1 (30 C, ~ 10.3 A g^-1), respectively. The excellent cycle stability and power capability are superior to most reported ZIBs or even some lithium-ion batteries. The results establish accurate electrochemical reaction mechanisms and kinetics for Zn/MnO₂, and identify the interplay of the voltage window and rate determining factors for achieving excellent cycle life.