N‑Substitution‑Directed Molecular Engineering of Organic Solid‑State Electrolytes for Potassium Ion Conduction

As a potential alternative and complement to lithium-ion batteries (LIBs), potassium-ion batteries (PIBs) draw global attention. However, the lack of high-performance potassium solid-state ion conductors is the core bottleneck impeding the practical application of all-solid-state PIBs. In this work, we employ an N-substitution strategy at a molecular level on potassium carbazolide (KCZ), where four N-substituted structural isomers are rationally fabricated. Among them, δ-potassium carbolinide (δ-KNCZ) demonstrates an ionic conductivity of 2.8 × 10^-4 S cm^-1 at 120°C with a transference number of 0.98, while the unsubstituted KCZ and other isomers (α-, β-, and γ-KNCZ) do not exhibit any detectable ionic conductivity. Crystal structure analysis reveals that δ-KNCZ crystallizes in an orthorhombic system with the space group of Pna2₁ (No. 33), in which K⁺ occupies a conduction-favorable planar quadrilateral coordination center with an elongated K─N ionic bond. Theoretical calculations further demonstrate that the K⁺ hops between tetra-coordinated centers with an energy barrier consistent with experimental results. Both experimental and theoretical results indicate that N-substitution at appropriate positions in organic substrates can tailor the coordination environment of K⁺ and weaken K─N ionic bonds, thereby facilitating potassium ion conduction. This study provides new insights into the design and development of organic solid-state ionic conductors.