Multifunctional Na2TiO3 Coating-Enabled High-Voltage and Capacitive-like Sodium-Ion Storage of Na0.44MnO2

Sodium-ion batteries, as an attractive option for large-scaleenergystorage, still face the problems of low energy density and unsatisfactoryrate performance. Among various cathodes, the tunnel-type Na0.44MnO2 with large S-shaped Na+ transport tunnelsis one of the promising cathode materials for fast and robust sodium-ionstorage, yet suffering from Mn dissolution and structural collapse.Herein, a Na-rich layered oxide Na2TiO3 is firstconstructed as a multifunctional coating layer on the surface of theNa(0.44)MnO(2) nanorod. Na2TiO3 not only acts as an Na+ reservoir, but also serves asa protective layer to prevent Na0.44MnO2 fromelectrolyte etching. Besides, the derived Ti-doped Na0.44MnO2 transition layer supplies additional Na+ diffusion pathways along the radial direction of the nanorod witha short migration distance. The optimized 3 wt % Na2TiO3-coated Na0.44MnO2 exhibits enhancedan initial capacity of 127 mAh g(-1) at 2-4.5V. In addition, it shows an ultra-high capacitive-like capacity ratioof 96.7%, hence delivering an excellent rate performance of 80.2 mAhg(-1) at 20C. Long-term cycling tests indicate splendidstability against high voltage, achieving 97.7% capacity retentionat 20C after 900 cycles. This work provides an effective strategyto improve the rate performance and high-voltage stability of Na0.44MnO2 for high energy and power density batteries.