A High-Temperature \beta-Phase NaMnO_2 Stabilized by Cu Doping and Its Na Storage Properties

The high-temperature \beta-phase NaMnO_2 is a promising material for Na-ion batteries (NIBs) due to its high capacity and abundant resources. However, the synthesis of phase-pure \beta-NaMnO_2 is burdensome and cost-ineffective 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 \beta 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 eV to 0.3 eV, which indicates that the electronic conductivity can be improved by Cu-doping. The designed \beta-NaCu_0.1Mn_0.9O_2 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.