Intrinsic linker flexibility in an anionic metal-organic framework electrolyte for selective lithium-ion conduction

Although solid-state batteries promise improved safety and higher energy density, their performance is fundamentally constrained by solid electrolytes that fail to reconcile fast, selective Li⁺ transport with conformal interfacial contact. Here, we report ZnBTCA, an anionic zinc-based metal-organic framework constructed from the flexible aliphatic linker 1,2,3,4-butanetetracarboxylic acid, introducing intrinsic framework compliance. Ion exchange converts Na⁺-ZnBTCA to its Li⁺ form, enabling highly Li⁺-selective ionic conduction within a mechanically soft framework (Young's modulus ≈ 4.6 GPa) that promotes conformal Li|electrolyte contact and reduces interfacial impedance. ZnBTCA exhibits Li⁺ conductivities of 3.53 × 10^-5 S cm^-1 at 20 °C and 1.87 × 10^-4 S cm^-1 at 60 °C (E _a = 0.36 eV), together with a high Li⁺ transference number (t _Li⁺ = 0.79). This combination of structural compliance and selective Li⁺ conduction enables stable interfacial behavior, demonstrated by ∼300 h of Li|ZnBTCA@5-PEO|Li cycling at 0.2-1.0 mA cm^-2 with stable voltage polarization. To our knowledge, this represents the first aliphatic-based MOF electrolyte and establishes linker flexibility coupled with framework anionicity as a general design strategy for mechanically adaptive, high Li⁺ transport MOF electrolytes in next-generation solid-state batteries.