Remarkable Improvements in Volumetric Energy and Power of 3D MnO 2 Microsupercapacitors by Tuning Crystallographic Structures

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 MnO 2 can be radically boosted by tuning crystallographic structures from cryptomelane (α‐MnO 2 ) to birnessite (δ‐MnO 2 ). As a result of greatly enhanced Na + accessibility and diffusion, 3D layered crystalline δ‐MnO 2 microelectrodes exhibit volumetric capacitance as high as ≈922 F cm −3 (≈1.5‐fold higher than α‐MnO 2 , ≈617 F cm −3 ) and excellent rate performance. This enlists δ‐MnO 2 microsupercapacitor to deliver ultrahigh stack electrical powers (up to ≈295 W cm −3 ) while maintaining volumetric energy density much higher than that of thin‐film lithium battery.