Li_2.9Fe_0.9Zr_0.1Cl₆ as Redox-Active Catholyte for Solid-State Li-Ion Batteries

Solid electrolytes are one of the key challenges that hinder the commercialization of all-solid-state batteries. Most efforts have been made to advance the development of solid electrolytes as separators, while the development of catholytes, particularly redox-active catholytes, has been less extensively studied. The high loading of catholytes in composite cathodes, while facilitating ionic conduction, drastically decreases the energy density of the battery. Here, we report an alternative strategy to improve the energy density by using Li_2.9Fe_0.9Zr_0.1Cl₆ as a redox-active catholyte. With a composite cathode containing uncoated LiCoO₂ and Li_2.9Fe_0.9Zr_0.1Cl₆, the solid-state cell not only shows excellent rate capability and stable long-term cycling, benefiting from the high ionic conductivity of Li_2.9Fe_0.9Zr_0.1Cl₆, but also shows a high cathode specific capacity of ∼153 mAh·g^-1. This study broadens the chemical space of the materials design for lithium-ion conductors with redox-active elements (e.g., Fe, Ti, V, and Cr), offering new opportunities to reduce the cost and improve the energy density for all-solid-state batteries.