Transition metal dichalcogenides like MoS2 can exist many phases like the semiconducting 2H and the metallic 1T phases which have shown intriguing properties for energy and electrocatalytic applications. However, the 2H and 1T phases normally distribute coherently in a single-layered MoS2 sheet which is accompanied with ubiquitous hetero-phase boundaries. In this work, by using density functional theory and electrochemical measurement, we report strong charge transfer ability between 2H/1T phase boundary of MoS2 and graphene which accounts for a superb coexistence of gravimetric and volumetric capacitances of 272 F g-1 and 685 F cm-3. As a proof-of-concept application, a flexible solid-state asymmetric supercapacitor based on MoS2/graphene is fabricated, showing a remarkable energy and power densities (46.3 mWh cm-3 and 3.013 Wcm-3). Our work shows the promise of promoting the efficiency of charge flow and energy storage through engineering phase boundary and interface in phase-change materials.