Ultrathin MgO coating on fabricated O3–NaNi0.45Mn0.3Ti0.2Zr0.05O2 composite cathode via magnetron sputtering for enhanced kinetic and durable sodium-ion batteries

An innovative and convenient method has been performed to stabilize the surface of O3-type NaNi0.45Mn0.3Ti0.2Zr0.05O2 composited electrode by ultrathin MgO film. There are few reports on the modification on the fabricated electrode instead of the active material powder, especially by radio frequency magnetron sputtering. Besides, the surface stability and capacity retention of MgO coated cathode are superior compared to the bare one under the high rate. The protective effect could physically enhance electrochemical performance because the MgO coating has been identified strictly. Three main reasons are discovered by investigating the mechanisms. Firstly, the MgO layer suppresses the dissolution of the active material into electrolyte; this might be the origin for more stable electrode structure. Secondly, the MgO layer boosts sodium-ion and electron transport owing to its location of the deposition. Finally, the appropriate thickness of MgO can minimize transmission resistance and SEI formation. Particularly, the half-cell constructed with NaNi0.45Mn0.3Ti0.2Zr0.05O2–MgO cathode delivers a good rate performance (80.3 and 70.3 mA h g−1 at 1200 and 1920 mA g−1) and a long cycle life (71.6 mA h g−1 at 240 mA g−1 for 300 cycles) when sputtering time is 10 s; meanwhile, the reversible discharge capacity (140.7 mA h g−1 at 12 mA g−1) with a cut-off voltage of 2–4 V is also a considerable achievement. The work provides a broad idea to apply MgO as a protective coating.