The high-temperature β-phase NaMnO2 is a promising material for Na-ion batteries (NIBs) due to its high capacity and abundant resources.However,the synthesis of phase-pure β-NaMnO2 is burdensome and costineffective because it needs to be sintered under oxygen atmosphere at high temperature and followed by a quenching procedure.Here we first report that the pure β phase can be stabilized by Cu-doping and easily synthesized by replacing a proportion of Mn with Cu via a simplified process including sintering in air and cooling to room temperature naturally.Based on the first-principle calculations,the band gap decreases from 0.7 e V to 0.3 e V,which indicates that the electronic conductivity can be improved by Cu-doping.The designed β-NaCu0.1Mn0.9O2 is applied as cathode in NIBs,exhibiting an energy density of 419 Wh/kg and better performance in terms of rate capability and cycling stability than those in the undoped case.