The pursuit of facile synthetic methods for systematic control over the morphology and crystal phase of nanostructures has attracted a tremendous amount of interest. By utilizing the acid-induced layer-to-tunnel transition method of MnO2, we here for the first time report multi-layered sawtooth-shape MnO2 with relatively high mass loading. The optimized electrode exhibits durable activity with enhanced surface area, electrical conductivity and ionic diffusion. A high areal capacitance of 1781.8 mF/cm(2) is obtained and excellent rate performance can be corroborated from capacitance retention of 64.1% when the current density is increased by 45-fold. Our uniquely assembled heterostructure endows a large number of active sites and acts as electron superhighways to facilitate efficient charge transportation. Further coupling with activated graphene/CC anode, a flexible device with a maximum operating voltage of 2.2 V is assembled, delivering a high volumetric energy density of 4.3 mWh/cm(3) at a power density of 27.6 mW/cm(3) with outstanding cycling performance. The finding promotes the development of highly efficient faradaic electrode materials toward closing the gap between achieved and theoretical capacitance without limiting the mass loading.