Abstract
Although O3-type layered oxides have become viable cathode materials for sodium-ion batteries (SIBs) due to their high energy densities, they still suffer from narrow ion channels and serious structure degradation, severely jeopardizing their electrochemical properties. Here, a stable O3-type layered oxide NaNi0.4Mn0.4M0.2O2 (M= Fe, Cu, Mg, Ti, Sn) (HE-NaNM) was synthesized by a high-entropy doping strategy. Owing to the introduction of the metal cations with ionic radiuses in a range of 0.61-0.73 Å into the layered structure, the lattice is expanded from 15.94 to 16.03 Å in c-axis, associated with a high distortion of TMO6 octahedrons, offering broad channels for sodium-ion transport in the lattices. Moreover, such expanded lattices and highly distorted TMO6 octahedrons enable to efficiently restrain the migration of TM ions and the severe sliding of TMO2 slabs, affording one-step reversible structure evolution between O3 and P3 phase during sodium extraction/insertion without formation of harmful interphases. As a result, a good rate capability of 77.9 mAh g-1 at 10 C and a long-term cycling stability up to 400 cycles at 5 C are achieved for sodium storage.
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