Ionic potential modulation within and between layers of transition metal oxides towards ultrahigh-rate sodium storage

Abstract

Although layered oxides have been considered promising cathodes for sodium-ion batteries (SIBs), they still suffer from poor structural stability and sluggish Na⁺ diffusion kinetics, hampering their cycling stability at high current rates. Herein, we demonstrate an efficient ionic potential modulation strategy to produce a lattice-stable layered oxide with favorable Na⁺ diffusion kinetics via selective introduction of cations with different ionic potentials into transition metal (TM) layers and Na layers of layered oxides. In contrast to the implantation of cations with high ionic potentials (Φ > 28.99 nm⁻¹), introducing cations with low ionic potentials (e.g., Li⁺, 13.16 nm⁻¹) into TM layers facilitates the delocalization of electron clouds around lattice O²⁻ towards TM ions, thereby constructing a unique O–TM–O interlocking configuration to simultaneously improve the stability of TM ions and lattice O²⁻. Meanwhile, the incorporation of cations with low ionic potentials such as K⁺ (7.25 nm⁻¹) into Na layers (Na⁺, 9.80 nm⁻¹) induces an attenuated K⁺–Na⁺ electrostatic interaction, thus diminishing the repulsion during the Na⁺ diffusion process. These unique features not only strengthen the skeletal structure of TM layers, but also promote the Na⁺ interlayer diffusion. Consequently, an excellent rate performance of 78.7 mA h g⁻¹ at 50 C and a long-term stability of up to 2000 cycles are achieved in SIBs.