Deciphering the Structure–Property Relationship of Na–Mn–Co–Mg–O as a Novel High-Capacity Layered–Tunnel Hybrid Cathode and Its Application in Sodium-Ion Capacitors

Developing novel cathode materials with a high energy density and long cycling stability is necessary for Na-ion batteries and Na-ion hybrid capacitors (NICs). Despite their high energy density, structural flexibility, and ease of synthesis, P-type Na layered oxides cannot be utilized in energy-storage applications owing to their severe capacity fading. In this regard, we report a novel composite layered–tunnel Na0.5Mn0.5Co0.48Mg0.02O2 cathode whose binary structure was confirmed via scanning electron microscopy and high-resolution transmission electron microscopy. Combination of the two-dimensional (2D) layered oxides with the three-dimensional tunnel structure, as well as the presence of Mg2+ ions, resulted in a high capacity of 145 mAh g–1 at a current density of 85 mA g–1, along with a high stability and rate capability. An NIC was fabricated with composite layered-tunnel structure as a battery-type electrode and commercial activated carbon as a counter electrode. The NIC exhibited a maximum energy density of 35 Wh kg–1 and good stability retaining 72% of its initial energy density after 3000 cycles. This integrated approach provides a new method for designing high-energy and high-power cathodes for NICs and NIBs.