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

Abstract

This work presents the study of the electronic structure of alluaudite-type Na_2.5Fe_1.75(SO₄)₃ and its impact on electrochemical performance as a cathode material for sodium-ion batteries (SIBs). Density functional theory calculations using the KKR-CPA method (Korringa–Kohn–Rostoker combined with the coherent potential approximation) revealed the diverse electrochemical activity of sodium ions occupying different sites within the alluaudite framework. Notably, an unprecedented contribution of sodium atoms to the overall electronic density of states near the Fermi level (dominated by Fe-d and O-p states) was observed – a feature not detected in layered transition metal oxide cathodes.

A high-purity Na_2.5Fe_1.75(SO₄)₃ cathode material was synthesized via an optimized solid-state route. Using a multi-technique approach – including X-ray diffraction, scanning electron microscopy, Mössbauer spectroscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and Raman spectroscopy – the evolution of the electronic structure and electrochemical behavior during sodium (de)intercalation was comprehensively characterized, providing deep insights into the sodium storage mechanism. Operando and in situ X-ray diffraction further tracked structural changes during cycling, showing that the material undergoes a reversible amorphization at deep sodium extraction.

Electrochemical tests demonstrated stable cycling with minimal capacity fade (only 2.5 % after 300 cycles at C/2), highlighting the high structural integrity and promise of this optimized cathode material.