A High-Temperature β-Phase NaMnO2 Stabilized by Cu Doping and Its Na Storage Properties**Supported by the National Key Technologies R&D Program of China under Grant No 2016YFB0901500, and the National Nature Science Foundation of China under Grant Nos 51725206 and 51421002.

Abstract

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 eV to 0.3 eV, which indicates that the electronic conductivity can be improved by Cu-doping. The designed β-NaCu0.1 Mn0.9O2 is applied as cathode in NIBs, exhibiting an energy density of 419Wh/kg and better performance in terms of rate capability and cycling stability than those in the undoped case.