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An innovative and convenient method has been performed to stabilize the surface of O3-type NaNi 0.45 Mn 0.3 Ti 0.2 Zr 0.05 O 2 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 NaNi 0.45 Mn 0.3 Ti 0.2 Zr 0.05 O 2 –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. • MgO film is performed via magnetron sputtering on NaNi 0.45 Mn 0.3 Ti 0.2 Zr 0.05 O 2 electrode. • The surface stability and capacity retention of MgO coated cathode are superior at high rate. • MgO layer boosts sodium-ion and electron transport owing to its location of deposition. • The appropriate thickness of MgO can minimize transmission resistance and SEI formation. • The NaNi 0.45 Mn 0.3 Ti 0.2 Zr 0.05 O 2 –MgO cell delivers 80.3 and 70.3 mA h g−1 at 1200 and 1920 mA g−1. [ABSTRACT FROM AUTHOR]

Due to the low cost and abundance of sodium, sodium-ion batteries (SIBs) show great potential as energy storage devices. As far as the current research is concerned, cathode materials are still the bottleneck for boosting the performance of SIBs. O3-type layered NaNi 0.5 Mn 0.5 O 2 is considered as an important cathode material of SIBs, but its irreversible phase transition, poor stability in air, and sluggish kinetics usually lead to reduced capacity, poor rate performance and high cost for material storage, which largely limit the large-scale application. Herein, a remarkable O3-type layered material NaNi 0.5-x Mn 0.3 Ti 0.2 Zr x O 2 (NaNMTZ, x = 0.02, 0.05), with stable phase structure has been synthesized by the solid-phase method. The partial substitution of electrochemical inactive Ti/Zr brings about higher electronic delocalization and entropy of mixing leading to enhanced structural stability. In addition, the enlarged interlayer spacing and good conductivity contribute to better rate capability. The micro-nano structure leads to faster Na+ diffusion rates and shorter diffusion paths for both ions and electrons. Therefore, NaNi 0.45 Mn 0.3 Ti 0.2 Zr 0.05 O 2 exhibits an initial reversible capacity of 141.4 mAh g−1 with a coulombic efficiency of 98.8% and remarkable capacity retention of 70% after 200 cycles at 0.05C, presenting better electrochemistry performance than the conventional NaNi 0.5 Mn 0.5 O 2. This study provides a new angle to design high capacity cathode materials with high rate capability and cycling stability through crystal lattice modulation. Image 1 • A remarkable O3-type layered NaNi 0.45 Mn 0.3 Ti 0.2 Zr 0.05 O 2 is designed as cathode material for sodium ion batteries. • The structural stability and rate performance of NaNi 0.45 Mn 0.3 Ti 0.2 Zr 0.05 O 2 are superior. • The higher electronic delocalization and better conductivity of NaNi 0.45 Mn 0.3 Ti 0.2 Zr 0.05 O 2 are provided. • The difference value about entropy of mixing between NaNi 0.5 Mn 0.5 and NaNi 0.45 Mn 0.3 Ti 0.2 Zr 0.05 O 2 is calculated. • The enlarged interlayer spacing and micro-nano structure contribute to the high rate capability. [ABSTRACT FROM AUTHOR]