Recently, heterostructures have attracted much attention in widespread research fields. By tailoring the physicochemical properties of the two components, creating heterostructures endows composites with diverse functions due to the synergistic effects and interfacial interaction. Here, a simple in situ localized phase transformation method is proposed to transform the transition-metal oxide electrode materials into heterostructures. Taking molybdenum oxide as an example, quasi-core-shell MoO3@MoO2heterostructures were successfully fabricated, which were uniformly anchored on reduced graphene oxide (rGO) for high-rate and highly durable lithium ion storage. The in situ introduction of the MoO2shell not only effectively enhances the electronic conductivity but also creates MoO3@MoO2heterojunctions with abundant oxygen vacancies, which induces an inbuilt driving force at the interface, enhancing ion/electron transfer. In operando synchrotron X-ray powder diffraction has confirmed the excellent phase reversibility of the MoO2shell during charge/discharge cycling, which contributes to the excellent cycling stability of the MoO3@MoO2/rGO electrode (1208.9 mAh g-1remaining at 5 A g-1after 2000 cycles). This simple in situ heterostructure fabrication method provides a facile way to optimize electrode materials for high-performance lithium ion batteries and possibly other energy storage devices.