Li<sub>2.9</sub>Fe<sub>0.9</sub>Zr<sub>0.1</sub>Cl<sub>6</sub> as Redox-Active Catholyte for Solid-State Li-Ion Batteries

Solid electrolytes\nare one of the key challenges that hinder the\ncommercialization of all-solid-state batteries. Most efforts have\nbeen made to advance the development of solid electrolytes as separators,\nwhile the development of catholytes, particularly redox-active catholytes,\nhas been less extensively studied. The high loading of catholytes\nin composite cathodes, while facilitating ionic conduction, drastically\ndecreases the energy density of the battery. Here, we report an alternative\nstrategy to improve the energy density by using Li<sub>2.9</sub>Fe<sub>0.9</sub>Zr<sub>0.1</sub>Cl<sub>6</sub> as a redox-active catholyte.\nWith a composite cathode containing uncoated LiCoO<sub>2</sub> and\nLi<sub>2.9</sub>Fe<sub>0.9</sub>Zr<sub>0.1</sub>Cl<sub>6</sub>, the\nsolid-state cell not only shows excellent rate capability and stable\nlong-term cycling, benefiting from the high ionic conductivity of\nLi<sub>2.9</sub>Fe<sub>0.9</sub>Zr<sub>0.1</sub>Cl<sub>6</sub>, but\nalso shows a high cathode specific capacity of ∼153 mAh·g<sup>–1</sup>. This study broadens the chemical space of the materials\ndesign for lithium-ion conductors with redox-active elements (e.g.,\nFe, Ti, V, and Cr), offering new opportunities to reduce the cost\nand improve the energy density for all-solid-state batteries.