Transition-metal oxides as faradaic charge-storage intermediates sandwiched between conductor and electrolyte are key components to store/deliver high-density energy in microsupercapacitors for many applications in miniaturized portable electronics and microelectromechanical systems. While the conductor facilitating their electron transports, they generally suffer from a switch of rate-determining step to their sluggish redox reactions in pseudocapacitive energy storage, during which poor cation accessibility and diffusion leads to high internal resistances and lowers volumetric capacitance and rate performance. Here it is shown that the faradaic processes in a model system of MnO2 can be radically boosted by tuning crystallographic structures from cryptomelane (alpha-MnO2) to birnessite (delta-MnO2). As a result of greatly enhanced Na+ accessibility and diffusion, 3D layered crystalline delta-MnO2 microelectrodes exhibit volumetric capacitance as high as approximate to 922 F cm(-3) (approximate to 1.5-fold higher than alpha-MnO2, approximate to 617 F cm(-3)) and excellent rate performance. This enlists delta-MnO2 microsupercapacitor to deliver ultrahigh stack electrical powers (up to approximate to 295 W cm(-3)) while maintaining volumetric energy density much higher than that of thin-film lithium battery.