Aqueous Zn-ion batteries (AZIBs) have been considered promising energy storage systems due to their low cost, high safety and environmental friendliness. Manganese dioxide (MnO₂) is a practically desirable cathode material for AZIBs; however, it is challenged by poor structural stability and unsatisfactory storage reversibility. Given the distinct advantages and limitations of single-phase MnO₂, Herein, constructing a dual-crystal-phases structure is proposed an effective strategy to comprehensively improve the storage capacity, rate capability, and cycling stability of AZIBs. {\mathrm{N}\mathrm{H}}_4^{+} cations are ingeniously introduced to the hydrothermal reaction system to precisely regulate crystalline phases of MnO₂. An optimal {\mathrm{N}\mathrm{H}}_4^{+} concentration endows the coexistence of α/δ-MnO₂ crystal phases with abundant heterogenous phase interfaces. The mismatch of the heterogenous crystal lattices results in abundant active structural defects at the dual-crystal-phases interfaces for efficient ionic storage and fast electronic/ionic transport. The heterogenous interfaces further enable structural stability of the MnO₂ cathode without structural deformation during cycling, thus enhancing the cycling performance of AZIBs. Electrochemical tests show that the α/δ-MnO₂ cathode provides a remarkable specific capacity of 297.6 mAh/g at 1 C, excellent rate performance (210.1 mAh/g at 3 C), and superior cycling stability (93.7% capacity retention after 600 cycles at 1 C). Moreover, flexible AZIBs based on the α/δ-MnO₂ cathode remain stable operation under bending conditions, demonstrating the practical potential of the α/δ-MnO₂ architecture. This study presents an innovative material design strategy for high-performance AZIB cathodes via dual-crystal-phase engineering, which can be extended to other electrode materials beyond AZIBs.