Computational Analysis of Mixed-Anion Inorganic Materials for Secondary Battery Cathode Materials
Authors: Kim Hyungjun , Casanova Adolfo Salgado , Wolverton Christopher M. 
Published: December 2024

For decades, battery design and development were based on the idea that the only possible source of redox in Li-ion cathode was on the transition metal. More recently, there has been exploration into the notion that anions could provide an extra capacity and energy density source for batteries. Nonetheless, anion redox is frequently linked with the irreversible discharge of oxygen from battery cathodes, presenting a notable obstacle to utilizing anion redox for creating enhanced batteries. Many studies have been conducted to effectively harness the reversible oxygen redox while preventing irreversible oxygen loss.1 However, various methods and their understandings are still insufficient.
In this situation, an anion substitution approach could be one of the remedies to utilize oxygen redox reversibility and understand its mechanism. Recently, Leube et al. reported on sulfur selenide mixed-anion cathode, which actually stored energy through stable anion redox reactions and also showed good cycle performance.2 We suggest that oxysulfides form a class of mixed-anion compounds that have properties making them potentially promising cathode materials. We apply isovalent anion substitution to oxide cathodes, with a fraction of oxygen ions replaced by sulfur ions. This isovalent substitution leaves the oxidation state of metals unchanged, thus having no impact on the overall quantity of available cation redox. At the same time, substituting sulfur ions allows us to regulate the anion p-states within the materials, which control the activity of anion redox in the system.
We first calculate the solubility limit of sulfur in oxide forming the solid solution oxysulfide. We model lithium cathode materials with oxygen and sulfur mixed in anion sites from 0 to 100%, based on structural prototypes known for anion redox behavior, considering 19 transition metals. We also model sulfur and selenium mixed cathodes, which have been experimentally reported.2 By comparing these two systems within the same prototype, we can identify the factors that make mixed-anion cathode difficult to synthesize. In fact, there are relatively few experimental or theoretical reports on oxysulfide synthesis. Therefore, this study aims to provide a theoretical interpretation of why it is challenging to form solid solution oxysulfide compared to sulfoselenide. We perform featurization on materials, including basic information, local and global structure features, etc. Using these features, we apply a machine learning technique to identify the key factors that hinder mixed-anion system synthesis. Our findings suggest the features that make it difficult to dissolve sulfur into oxide, offering a theoretical guide for constructing other types of mixed-anion systems.

Reference

A. S. Menon, M. J. W. Ogley, A. R. Genreith-Schriever, C. P. Grey, and L. F. J. Piper, Annu Rev Mater Res, 54, 199–221 (2024)

B. Leube, C. Robert, D. Foix, B. Porcheron, R. Dedryvère, G. Rousse, E. Salager, P. Cabelguen, A. Abakumov, H. Vezin, M. Doublet, J. Tarascon, Nature Communications 2021 12:1, 12, 1–11 (2021)