Bridging Transition Metal and Anion Redox Processes in Li-Rich Sulfide Cathodes

Li-ion batteries are essential for decarbonizing global transport and energy, but their scalability is constrained by limited supplies of critical cathode elements, such as Ni, Mn, Co, and P. To address this, we previously introduced high-energy-density Li-ion cathodes composed of Al, Fe, and S, which are elements already produced globally at industrial scale and battery-grade purity. These cathodes leverage sulfide anion redox, involving nonbonding S 3p states and localized distortions that form and break S-S bonds, enabling high capacity. Here, we expand this chemical space by incorporating Cu into cathodes Li_{2.2- z} Cu _z Al_0.2Fe_0.6S₂ (0 ≤ z ≤ 0.4), where highly covalent Cu-S interactions stabilize holes on Cu as Cu^>1+. This Cu redox extends charge compensation that was previously restricted to localized, electronically isolated S-S bonds. Cu also limits capacity, which we attribute to structural destabilization of the delithiated phase, despite the thermodynamic stability of Cu^>1+. By describing the effects of Cu on charge compensation and phase stability, we present a sulfide anion redox mechanism for next-generation multielectron redox Li-ion cathodes, where highly covalent transition metal states participate in otherwise electronically isolated redox processes involving anion nonbonding states.