As a crucial part of electric energy storage devices, nickel-zinc alkaline aqueous batteries have received increasing research interests. However, the low energy/power densities and limited stability seriously impede their large-scale application in electrochemical energy storage. Herein, a simple hydrothermal method is used to synthesize uniform single-crystal β-Ni(OH)₂ quasi-nanocubes (β-NQNCs). And the β-NQNC cathode is used for developing stable aqueous Ni-Zn battery with good energy density. The nanosized β-NQNCs with single-crystal structure can enhance the stability of the cathode and speed up the charge transfer process. The exposed polar (001) crystal facets possess high surface energy and high reactivity. The adsorption of SO2-4 ions may be one possibility for the preparation of β-NQNCs. And the prepared β-NQNC//Zn battery shows ideal areal specific capacity (0.34 mA h cm^-2 at 0.36 mA cm^-2), areal energy density (0.592 mW h cm^-2) and power density (5.612 mW cm^-2). More importantly, the battery exhibits good flexibility under bending. This work may provide new insights into designing aqueous rechargeable batteries with good energy density of nano-single-crystal electrodes.