Dense (${\text{Na}}_{0.5}$${\text{K}}_{0.5})$NbO₃ lead-free ceramics with the simple composition were prepared via sintering in low oxygen partial pressure (pO₂, $\sim$${\text{10}}^{-12}$ atm) atmosphere and adding LiF. All the ceramics have pure orthorhombic structure. Compared to the LiF-added (${\text{Na}}_{0.5}$${\text{K}}_{0.5})$NbO₃ ceramics sintered in air and the low pO₂-sintered pure (${\text{Na}}_{0.5}$${\text{K}}_{0.5})$NbO₃ ceramics without LiF addition, the present ceramics exhibit improved piezoelectric and ferroelectric properties. The piezoelectric constant $d_{33}$ is 125 pC/N, and the converse piezoelectric constant $d_{33}$* is 186 pm/V. The dielectric constant and dielectric loss of the ceramics at room temperature and 1 kHz are 451 and 0.03, respectively. Under the measured electric field of 70 kV/cm, the remanent polarization is 25.9 $\mu$C/cm² and the coercive field is 13.9 kV/cm. Furthermore, if the base metals such as Cu and Ni powders were mixed into the green pellets and sintered in the low pO₂ atmosphere, the base metals cannot be oxidized, suggesting possibility of using base metals as electrodes.Dense (${\text{Na}}_{0.5}$${\text{K}}_{0.5})$NbO₃ lead-free ceramics with the simple composition were prepared via sintering in low oxygen partial pressure (pO₂, $\sim$${\text{10}}^{-12}$ atm) atmosphere and adding LiF. All the ceramics have pure orthorhombic structure. Compared to the LiF-added (${\text{Na}}_{0.5}$${\text{K}}_{0.5})$NbO₃ ceramics sintered in air and the low pO₂-sintered pure (${\text{Na}}_{0.5}$${\text{K}}_{0.5})$NbO₃ ceramics without LiF addition, the present ceramics exhibit improved piezoelectric and ferroelectric properties. The piezoelectric constant $d_{33}$ is 125 pC/N, and the converse piezoelectric constant $d_{33}$* is 186 pm/V. The dielectric constant and dielectric loss of the ceramics at room temperature and 1 kHz are 451 and 0.03, respectively. Under the measured electric field of 70 kV/cm, the remanent polarization is 25.9 $\mu$C/cm² and the coercive field is 13.9 kV/cm. Furthermore, if the base metals such as Cu and Ni powders were mixed into the green pellets and sintered in the low pO₂ atmosphere, the base metals cannot be oxidized, suggesting possibility of using base metals as electrodes.