Multifunctional Na₂TiO₃ Coating-Enabled High-Voltage and Capacitive-like Sodium-Ion Storage of Na_0.44MnO₂

Sodium-ion batteries, as an attractive option for large-scale energy storage, still face the problems of low energy density and unsatisfactory rate performance. Among various cathodes, the tunnel-type Na_0.44MnO₂ with large S-shaped Na⁺ transport tunnels is one of the promising cathode materials for fast and robust sodium-ion storage, yet suffering from Mn dissolution and structural collapse. Herein, a Na-rich layered oxide Na₂TiO₃ is first constructed as a multifunctional coating layer on the surface of the Na_0.44MnO₂ nanorod. Na₂TiO₃ not only acts as an Na⁺ reservoir, but also serves as a protective layer to prevent Na_0.44MnO₂ from electrolyte etching. Besides, the derived Ti-doped Na_0.44MnO₂ transition layer supplies additional Na⁺ diffusion pathways along the radial direction of the nanorod with a short migration distance. The optimized 3 wt % Na₂TiO₃-coated Na_0.44MnO₂ exhibits enhanced an initial capacity of 127 mAh g^-1 at 2-4.5 V. In addition, it shows an ultra-high capacitive-like capacity ratio of 96.7%, hence delivering an excellent rate performance of 80.2 mAh g^-1 at 20C. Long-term cycling tests indicate splendid stability against high voltage, achieving 97.7% capacity retention at 20C after 900 cycles. This work provides an effective strategy to improve the rate performance and high-voltage stability of Na_0.44MnO₂ for high energy and power density batteries.