Microsphere Na_0.65[Ni_0.17Co_0.11Mn_0.72]O₂ Cathode Material for High-Performance Sodium-Ion Batteries

P2-type layered oxides have been considered promising candidates as cathodes for sodium-ion batteries (SIBs) owing to their high capacity and high rate capability. However, because of the difficulty involved in forming hierarchical microstructures, it remains challenging to develop high energy density P2-type layered oxides with good electrochemical performance and high electrode density. In this study, we demonstrate the feasibility of P2-type Na_0.65[Ni_0.17Co_0.11Mn_0.72]O₂ as a very efficient cathode material for high energy density SIBs by synthesizing a micron-sized hierarchical structure via the coprecipitation route. The as-prepared P2-type microsphere cathode constructed from nanoscale primary particles provides a sufficient interface between the electrodes and the electrolyte solution, which enables to shorten the transport pathways for Na⁺ ions and electrons. Simultaneously, the hierarchical microstructure enhances the structural stability and high tap density (∼1.18 g cm^-3). Benefiting from these merits, the proposed P2-type microsphere Na_0.65[Ni_0.17Co_0.11Mn_0.72]O₂ displays a high discharge capacity of 187 mA h g^-1 at 12 mA g^-1 and an exceptional cycle retention of 74.7% after 500 cycles, even at the high current density of 600 mA g^-1. In addition, the high tap density of this P2-type microsphere enhances the density of composite cathodes, which translates to a high volumetric energy density of 340 W h L^-1 based on the overall volume of the cathode active mass and the aluminum foil current collector.