A Single-Phase Mixed Ion-Electron Conducting Metal–Organic Framework

Mixed ionic-electronic conductors (MIECs) are highly sought after for electrochemical systems because they support concurrent charge and mass transport. Yet, structurally well-defined single-phase MIECs remain scarce, as most systems rely on physical mixtures of ionic and electronic conductors. Here, we introduce a cation-rich design strategy to realize solid-state mixed Li⁺-electronic conduction in a two-dimensional copper–catecholate metal–organic framework, Cu₃(HOTAT)₂, built from the 3-fold symmetric new ligand 2,3,7,8,12,13-hexahydroxytriazatruxene (HHTAT). Owing to the combined redox activity of Cu²⁺/Cu⁺ and the HOTAT ligand, controlled fractional reduction generates a family of Li_xCu₃(HOTAT)₂ (0 ≤ x ≤ 7.50) phases with tunable transport properties, in good agreement with electronic-structure calculations. The Li-rich phase Li_7.50Cu₃(HOTAT)₂ exhibits intrinsic mixed conduction at room temperature, with an electronic conductivity of 2.8 × 10^–3 S cm^–1, and solid-state Li⁺ conductivity of 1.1 × 10^–3 S cm^–1. As a proof of concept, Li_7.50Cu₃(HOTAT)₂ operates as a homogeneous cathode in all-solid-state Li batteries, delivering 100 mAh g^–1 after 100 cycles with ∼99.8% Coulombic efficiency, indicative of highly reversible electrochemical behavior. These results establish cation-rich reduction of redox-active 2D MOFs as an efficient route to engineer solid-state mixed Li⁺-electronic conductors, opening a pathway toward dual-conducting porous materials for solid-state electrochemical technologies.