Single-Ion Li⁺, Na⁺, and Mg²⁺ Solid Electrolytes Supported by a Mesoporous Anionic Cu-Azolate Metal-Organic Framework

Area of Science:

• Materials Science
• Inorganic Chemistry
• Electrochemistry

Background:

• Metal-organic frameworks (MOFs) offer tunable porous structures for various applications.
• Developing efficient solid-state electrolytes is crucial for next-generation energy storage devices.
• Existing MOF-based electrolytes face challenges in ion conductivity and loading capacity.

Purpose of the Study:

• To synthesize and characterize a novel Cu(II)-azolate MOF with tubular pores.
• To investigate the MOF's reversible single crystal to single crystal transition.
• To explore the MOF's potential as a high-performance solid-state electrolyte for Li+, Na+, and Mg2+ ions.

Main Methods:

• Single crystal X-ray diffraction to study structural transitions.
• Stoichiometric reaction with halide/pseudohalide salts for anion exchange.
• Ion loading with Li+, Na+, and Mg2+ salts.
• Electrochemical impedance spectroscopy to measure ionic conductivity.

Main Results:

• A novel Cu(II)-azolate MOF exhibiting reversible phase transitions was successfully synthesized.
• The MOF demonstrated stoichiometric loading of record amounts of Li+, Na+, and Mg2+ ions.
• Single-ion conductivity was achieved due to stationary anions and mobile cations within 1D pores.
• Achieved ionic conductivities: 4.4 × 10^-5 S/cm (Li+), 1.8 × 10^-5 S/cm (Na+), and 8.8 × 10^-7 S/cm (Mg2+).
• Li+ conductivity enhanced to 4.8 × 10^-4 S/cm upon addition of LiBF4.

Conclusions:

• The novel MOF serves as an efficient platform for developing high-performance solid-state electrolytes.
• The reversible phase transition mechanism facilitates high ion loading capacity.
• The observed ionic conductivities represent the highest values reported for MOF solid electrolytes to date.
• This work paves the way for advanced MOF-based energy storage applications.