A tetraphenylborate-based anionic metal–organic framework as a versatile solid electrolyte for fast Li+, Na+, K+, Mg2+, Ca2+, and Zn2+ transportation

Abstract

Tetraphenylborate (BPh₄⁻) has been widely employed in the field of electrolytes and displayed better ionic conductivities in polymer solid-state Li⁺ conductors. However, the fabrication of tetraphenylborate monomers into metal–organic frameworks (MOFs) and the exploration of their potential in solid-state electrolytes have never been reported. In this work, carboxylic acid functionalized lithium tetraphenylborate was purposefully synthesized and employed to construct an anionic MOF as a solid electrolyte. The counter cation Li⁺ was encapsulated into the anionic channel to become the free mobile charge carrier that produced a lithium-ion solid electrolyte with outstanding ion conductivity (2.75 × 10⁻³ S cm⁻¹ at 25 °C), an impressively high lithium-ion transference number (t_Li⁺ = 0.89), and low activation energy (0.15 eV). Acting as a solid electrolyte, the anionic MOF-based lithium iron phosphate battery delivered an initial specific capacity of 135 mA h g⁻¹ and retained 95% capacity after 220 charge–discharge cycles with a coulombic efficiency close to 100%. Moreover, by exchanging the free Li⁺ with Na⁺, K⁺, Mg²⁺, Ca²⁺, and Zn²⁺, our anionic MOF is also available for other types of solid electrolytes with the corresponding conductivities all above that of the functional battery electrolyte. Our work provided a convenient and tunable route to prepare conducting MOFs for alkali metal ions, alkaline earth metal ions, and other possible metal cations of interest, which could be used in solid-state electrolytic devices in the future.