A Metal-Organic Framework of Organic Vertices and Polyoxometalate Linkers as a Solid-State Electrolyte

A new three-dimensional metal-organic framework (MOF) was synthesized by linking ditopic amino functionalized polyoxometalate [N(C₄H₉)₄]₃[MnMo₆O₁₈{(OCH₂)₃CNH₂}₂] with 4-connected tetrahedral tetrakis(4-formylphenyl)methane building units through imine condensation. The structure of this MOF, termed MOF-688, was solved by single crystal X-ray diffraction and found to be triply interpenetrated diamond-based dia topology. Tetrabutylammonium cations fill the pores and balance the charge of the anionic framework. They can be exchanged with lithium ions to give high ionic conductivity (3.4 × 10^-4 S cm^-1 at 20 °C), a high lithium ion transference number (t_Li+ = 0.87), and low interfacial resistance (353 Ω) against metallic lithium-properties that make it ideally suited as a solid-state electrolyte. Indeed, a prototype lithium metal battery constructed using MOF-688 as the solid electrolyte can be cycled at room temperature with a practical current density of ∼0.2 C.