Alkali-independent Anion Redox in LiNaFeS₂

Although Na-ion batteries (SIBs) present a promising and more sustainable solution compared to Li-ion batteries, SIBs have comparatively lower energy density and suffer from irreversible charge storage due to the size and mass of Na+. In recent years, the investigation of Li-rich materials has revealed employing multielectron redox that couples cation and anion redox is a method to improve capacity. Coupling anion and cation redox could be a way to improve the low energy density of Na-ion cathodes, but the influence of the large Na+ on these electrochemical processes is not well understood. Here, we leverage the mixed-alkali nature of LiNaFeS2 to compare its behavior in Li vs Na half cells. LiNaFeS2 is known to support multielectron redox by virtue of cation and anion redox in Li half cells. We now demonstrate that LiNaFeS2 can also be used as a multielectron Na cathode. Elemental analysis of the cathodes at various states of charge verify Na as the dominant mobile ion after first charge, which correlates to deviations in the electrochemistry compared to the material cycled in a Li half cell. Notably, cycling with a Na electrolyte causes the particles to roughen and amorphize. Fe and S K-edge X-ray absorption spectroscopy show that the charge compensation mechanisms from an electronic structure point of view are fundamentally the same independent of cell configuration. Significant structural differences between the Li and Na half cells are observed and further probed with synchrotron X-ray diffraction studies, scanning electron microscopy, and cyclic voltammetry coupled with b-value analysis. Our work provides a fundamental study on the differences between Li- and Na-based anion redox in the same material.