Work

Unique properties of the electronic structure of alluaudite sodium iron sulfate cathode material and the impact on its electrochemical performance

Year: 2025

Type: article

Abstract: This work explores the unique properties of the electronic structure of alluaudite-type Na2.5Fe1.75(SO4)3, which impacts its electrochemical properties as a cathode for sodium-ion batteries (SIBs). Density functional theory calculations using the implemented KKR-CPA package (Korringa-Kohn-Rostoker method combined with the coherent potential approximation) showed the diverse electrochemical activity of sodium at different sites within the alluaudite structure. This revealed unprecedented contributions of the Na atoms to the overall electronic density of states near the Fermi energy (dominated by d-Fe and p-O states), undetected before in the group of layered transition oxide cathode materials. A high-purity Na2.5Fe1.75(SO4)3 cathode material was synthesized by optimizing the solid-state synthesis route. Based on a multi-technique approach including: X-ray diffraction, scanning electron microscopy, Mössbauer, X-ray photoelectron, Fourier Transform Infrared, and Raman spectroscopy studies, the electronic structure and electrochemical properties of the cathode material and its evolution during the sodium deintercalation/intercalation processes have been characterized, providing a comprehensive understanding of the sodium storage mechanism. Operando and in-situ X-ray diffraction studies monitored changes in the crystal structure of the cathode material during charge/discharge cycles showing that the system exhibits a reversible tendency to amorphize at deep sodium deintercalation. The electrochemical analysis showed reversible cycling with minimal capacity loss (only 2.5% after 300 cycles at discharge at C/5 current loading), demonstrating high structural stability of this optimized cathode material.

Source: Acta Materialia

DOI: 10.1016/j.actamat.2025.121582