Synthesis and electrochemical behaviors of layered Na-0.67[Mn0.65Co0.2Ni0.15]O-2 microflakes as a stable cathode material for sodium-ion batteries

Stable Na+ ion storage cathodes with adequate reversible capacity are now greatly needed for enabling Na-ion battery technology for large scale and low cost electric storage applications. In light of the superior Li+ ion storage performance of layered oxides, pure P2-phase Na-0.67[Mn0.65Ni0.15Co0.2]O-2 microflakes are synthesized by a simple sol-gel method and tested as a Na+ ion storage cathode. These layered microflakes exhibit a considerably high reversible capacity of 141 mA h g(-1) and a slow capacity decay to 125 mA h g(-1) after 50 cycles, showing much better cyclability than previous NaMnO2 compounds. To further enhance the structural and cycling stability, we partially substituted Co3+ by Al3+ ions in the transition-metal layer to synthesize Na-0.67[Mn0.65Ni0.15Co0.15Al0.05]O-2. As expected, the Al-substituted material demonstrates a greatly improved cycling stability with a 95.4% capacity retention over 50 cycles, possibly serving as a high capacity and stable cathode for Na-ion battery applications.