Hollow structured medium entropy transition metal selenide CoNiFe-Se@NC enables high performance of sodium-ion batteries

Abstract

Entropy engineering holds great promise in enhancing the ion conductivity of the electrode of sodium-ion batteries during charge and discharge cycles. However, it remains challenging to fabricate hollow structures for medium and high entropy nanomaterials due to the complicated chemistry. Herein, a hollow-structured medium entropy (HSME) Fe/Co/Ni ternary transition metal selenide wrapped in a carbon shell (CoNiFe-Se@NC), was deliberately designed and prepared by sequential ion exchange of ZIF-67 with other Ni²⁺ and Fe²⁺ metal ions. CoNiFe-Se@NC exhibits a reduced sodium ion diffusion energy barrier and enhanced sodium ion diffusion rate compared to its binary counterparts, as demonstrated by theoretical calculation and experimental results. When applied as an anode material for sodium-ion batteries, the hollow-structured CoNiFe-Se@NC demonstrates outstanding rate performance (346.8 mA h g⁻¹ at 20 A g⁻¹) and ultra-long cycle stability (349.5 mA h g⁻¹ after 5000 cycles at 10 A g⁻¹) as an anode material for sodium-ion batteries, and maintains a high specific capacity of 264.5 mA h g⁻¹ after 200 cycles at a current density of 1 A g⁻¹ in a full sodium-ion cell paired Na₃V₂(PO₄)₃@C cathode. This work highlights the importance of designing hollow structures and entropy engineering, and may shed light on the preparation of high-performance sodium-ion electrodes.