High entropy induced lattice expansion in layered oxide cathode towards fast sodium storage

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 NaNi_0.4Mn_0.4M_0.2O₂ (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.