The global demand for energy storage technology has created a tremendous need for Li-ion batteries that are lightweight, low cost, and sustainable. Research in the field of lithium-rich cathode materials has led to the development a new class of lithium-excess, cation-disordered rocksalt (DRX) materials. DRX materials display exciting promise as potential cathodes in lithium-ion batteries, owing to their high capacity and resource-friendly composition. Despite these exciting characteristics, these materials suffer from capacity loss over extended cycling due to deleterious side reactions that occur at high potentials, such as O_2 loss. It has been shown that oxygen loss can be suppressed by partial substitution of the lattice oxygen for fluorine, but the role of fluorine in the material remains unclear. In this study, we examine the impact of fluorination on the anionic reactivity of DRX materials. We first use Differential Electrochemical Mass Spectrometry (DEMS) and Titration Mass Spectrometry to map first-charge electrochemistry in DRX cathodes. Comparing the results of this analysis for a DRX oxide and a DRX oxyfluoride, we observe that fluorination increases the transition metal capacity and reduces the oxygen redox capacity. These two effects influence the total material capacity in opposite ways, shifting the balance between transition metal redox and oxygen redox without strongly affecting the total charge capacity. We also adapt an existing technique that couples DEMS with a fluoride-scavenging additive, using it for the first time to show that small amounts of fluorine dissolve from DRX oxyfluorides during charging to high potentials. Finally, we extend these techniques over the first several cycles to study the reversibility of the redox processes and the stability of the materials during cycling. We demonstrate that while oxygen redox remains mostly reversible from cycle to cycle, electrolyte degradation and fluoride dissolution also continue to occur to a diminishing extent during cycling. These conclusions motivate surface passivation to control interfacial reactivity as an important direction to improve cycling stability.
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