Synergistic High-Entropy Engineering in Biphasic Layered Oxides Enables High-Rate Sodium-Ion Cathodes

Abstract

Layered oxide cathodes for sodium-ion batteries (SIBs) are plagued by irreversible oxygen redox and structural degradation at high voltages, leading to severe capacity fade. To address this challenge, we synthesized a high-entropy biphasic oxide cathode, P2/O3-Na0.6Mn0.44Ni0.2Fe0.09Cu0.09Ti0.1Li0.04Mg0.04O2 (MNFCTLM) via solid-phase method. The high-entropy composition effectively stabilizes the transition metal (TM)-O bonds, suppresses oxygen activity, and mitigates O2 precipitation. Concurrently, the biphasic structure offers additional Na+ migration channels, significantly enhancing ion diffusion kinetics. As a result, the MNFCTLM cathode delivers a specific capacity of 134.5 mAh g−1 at 0.2C and exhibits exceptional cycling stability, retaining 81.3% of its capacity after 200 cycles at 2C. It also achieves an excellent rate performance (83.2 mAh g−1 at 20C) and exceptional air stability, maintaining 86.4% of its initial capacity after seven days of air exposure. Ex situ XPS analysis revealed that, compared to the low-entropy P2-Na0.6Mn0.7Ni0.21Fe0.09O2 (MNF) cathode, MNFCTLM suppresses the formation of unstable On- species at high voltage. Ex situ XRD combined with DFT calculations further demonstrated the superior structural stability of MNFCTLM. In a full cell with a hard carbon anode, it demonstrates a high capacity of 160 mAh g−1 at 0.2C and maintains 81.2% capacity after 200 cycles at 2C. This work not only presents a high-performance cathode for SIBs but also provides a fundamental insight into stabilizing anionic redox through entropy engineering, offering a generalizable design strategy for advanced layered oxide electrodes.