Chemical Design of IrS2 Polymorphs to Understand the Charge/Discharge Asymmetry in Anionic Redox Systems

Li-ion batteries are growing in demand and such growth calls for the quest for high-energy-density electrode materials. Li-rich layered oxides that show both cationic and anionic redox are expected to meet the high energy requirement. However, the oxygen anion activity triggers numerous structural and electronic rearrangements that need to be understood prior to envisioning applications. Here, we chemically design two new LixIrS2 polymorphs to further interrogate the mechanisms of the ligand redox process. By combined structural and spectroscopic characterizations, we show that electrochemical lithiation/delithiation of the polymorphs involve different sulfur redox couples that stand as unusual behavior. These structure-dependent kinetic pathways lead to an similar to 1 V difference between the two polymorphs, hence providing the missing link between the structure and hysteresis in anionic redox systems. These insights into the origin of hysteresis can guide proper parameters to cure it, hence laying the groundwork for the design of new practical electrode materials.