Alkali-independent\nAnion Redox in LiNaFeS<sub>2</sub>

Although Na-ion batteries\n(SIBs) present a promising and more sustainable\nsolution compared to Li-ion batteries, SIBs have comparatively lower\nenergy density and suffer from irreversible charge storage due to\nthe size and mass of Na<sup>+</sup>. In recent years, the investigation\nof Li-rich materials has revealed employing multielectron redox that\ncouples cation and anion redox is a method to improve capacity. Coupling\nanion and cation redox could be a way to improve the low energy density\nof Na-ion cathodes, but the influence of the large Na<sup>+</sup> on\nthese electrochemical processes is not well understood. Here, we leverage\nthe mixed-alkali nature of LiNaFeS<sub>2</sub> to compare its behavior\nin Li vs Na half cells. LiNaFeS<sub>2</sub> is known to support multielectron\nredox by virtue of cation and anion redox in Li half cells. We now\ndemonstrate that LiNaFeS<sub>2</sub> can also be used as a multielectron\nNa cathode. Elemental analysis of the cathodes at various states of\ncharge verify Na as the dominant mobile ion after first charge, which\ncorrelates to deviations in the electrochemistry compared to the material\ncycled in a Li half cell. Notably, cycling with a Na electrolyte causes\nthe particles to roughen and amorphize. Fe and S K-edge X-ray absorption\nspectroscopy show that the charge compensation mechanisms from an\nelectronic structure point of view are fundamentally the same independent\nof cell configuration. Significant structural differences between\nthe Li and Na half cells are observed and further probed with synchrotron\nX-ray diffraction studies, scanning electron microscopy, and cyclic\nvoltammetry coupled with <i>b</i>-value analysis. Our work\nprovides a fundamental study on the differences between Li- and Na-based\nanion redox in the same material.