XPS Analysis of Chemical Composition, Redox Mechanisms, and Deintercalation Pathways in Alluaudite-type Sodium-Ion Battery Cathodes

Sodium-ion batteries (SIBs) offer a versatile and scalable energy storage solution for various applications, including grid storage and mobile devices. In this contribution, we focus on investigating the sodium deintercalation pathways in alluaudite-type Na_2.5-xFe_1.75(SO₄)₃ cathode material using X-ray photoelectron spectroscopy (XPS), providing surface-sensitive insights into its chemical composition, as well as the chemical states and local environments of the elements under investigation at several charging potentials. These studies are complemented by theoretical investigations and numerous experimental techniques, including X-ray diffraction, Mössbauer spectroscopy, Raman spectroscopy, infrared spectroscopy, and electrochemical analysis. Na 1s core-level analysis indicates the presence of several distinct sodium components, attributed to different chemical environments within the cathode material. Analyzing the respective core-level peak areas upon charging sheds light on the desodiation process, suggesting varied chemical behavior among the components and indicating that it may be driven by local coordination asymmetry, site-dependent activation energy, and iron cation migration. The change in this latter component, associated with Na⁺ occupying Fe sites, is less prominent, consistent with its electrochemical inactivity. Complementary Fe 2p₃/₂ XPS analysis, based on a multiplet splitting approach, confirms gradual oxidation from Fe²⁺ to Fe³⁺ and reveals a correlation between charge compensation and Na⁺ loss. The deviation from ideal charge balance is attributed to excess Na⁺ resulting from off-stoichiometry, as well as contributions from surface layers and interfacial reactions, as indicated by detailed C 1s XPS analysis. Together, these findings suggest a kinetically regulated Na⁺ deintercalation mechanism shaped by Fe migration and site-dependent electrostatic and electronic environments, providing insights into the surface chemistry and structural evolution of SIBs.

This work is financially supported by German Research Foundation (DFG, project number 504885810) within the OPUS-22 (LAP) program, and Polish National Science Center (NCN, OPUS 22 LAP project number: 2021/43/I/ST8/01125, AGH Excellence Initiative – Research University action 4 no. 9805