Synthesis and electrochemical behaviors of layered Na0.67[Mn0.65Co0.2Ni0.15]O2 microflakes as a stable cathode material for sodium-ion batteries

Abstract

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[Mn_0.65Ni_0.15Co_0.2]O₂ 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[Mn_0.65Ni_0.15Co_0.15Al_0.05]O₂. 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.