One-step facile synthesis of microporous mix-crystals of α−, β-, and γ-Bi2O3 is proposed by the thermal decomposition of bismuth nitrate, and the microporous Bi2O3 mix-crystals are characterized by scanning electron microscope, X-ray powder diffractometer, Fourier transform infrared spectrometer, thermogravimeter and differential scanning calorimeter, and Brunauer–Emmett–Teller surface area measurements. A long life Li ion full battery is constructed with the microporous Bi2O3 mix-crystals as cathode and the porous lithium foil/Li-graphite as anode. The full battery is tested by the galvanostatic current charge-discharge technology at higher current densities of 0.5 mA cm–2. The microporous Bi2O3 mix-crystals and lithium foil-graphite can effectively improve the discharge capacity and cycle life of full battery.