High-Performance Cathode of Sodium-Ion Batteries Enabled by a Potassium-Containing Framework of K_0.5Mn_0.7Fe_0.2Ti_0.1O₂

Sodium-ion batteries (SIBs) are promising candidates for large-scale electric energy storage with abundant sodium resources. However, their development is challenged by the availability of satisfactory cathode materials with stable framework to accommodate the transportation of large-sized Na⁺ (1.02 Å), whose continuous insertion/extraction can easily cause irreversible volumetric deformation in the crystalline material, leading to inevitable structural failure and capacity fading. Here, different from the previous synthesis efforts targeting at Na⁺ containing compounds, we unveil the possibility of achieving a highly reversible sodiation/desodiation process by resorting to a K⁺-based layered metal oxide formulated as K_0.5Mn_0.7Fe_0.2Ti_0.1O₂ (KMFT), which is a P2 type in structure with a wide interlayer spacing to sit K⁺ (1.38 Å). We demonstrate that an initial K⁺/Na⁺ exchange can introduce Na⁺ into the lattice while a small amount of K⁺ remains inside, which plays a significant role in ensuring enlarged channels for a fast and stable Na⁺ diffusion. The KMFT electrode delivers a high initial discharge capacity of 147.1 mA h g^–1 at 10 mA g^–1 and outstanding long cycling stability with capacity retention of 71.5% after 1000 cycles at 500 mA g^–1. These results provide a new design strategy for the development of stable SIBs cathodes to facilitate their future applications.