Air-Stable Membrane-Free Magnesium Redox Flow Batteries

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Membrane-free biphasic self-stratified batteries (MBSBs) offer manufacturing flexibility and cost-effectiveness.
• Aqueous/nonaqueous electrolyte systems are key to developing advanced battery technologies.
• Magnesium (Mg) metal anodes are promising for high-energy-density batteries.

Purpose of the Study:

• To develop an ultrastable, high-voltage Mg-based MBSB using an aqueous/nonaqueous electrolyte system.
• To evaluate the performance of Mg MBSBs with metal-free catholytes (TEMPO and C3-PTZ) under various conditions.
• To demonstrate the potential of Mg chemistry in organic flow batteries.

Main Methods:

• Engineered an aqueous electrolyte with a 3.24 V electrochemical stability window.
• Developed a Mg metal anode with a Mg²⁺-conductive protective coating.
• Utilized 2,2,6,6-tetramethylpiperdinyl oxy (TEMPO) and N-propyl phenothiazine (C3-PTZ) as metal-free catholytes.
• Tested Mg||TEMPO and Mg||C3-PTZ MBSBs under static, stirred, and flow conditions.

Main Results:

• Achieved high cell voltages of 2.07 V (Mg||TEMPO) and 2.12 V (Mg||C3-PTZ).
• Static batteries (0.5 M) showed >97% capacity retention over 500 cycles at 8 mA/cm².
• Flow batteries achieved power densities of 195 mW/cm² (Mg||TEMPO) and 191 mW/cm² (Mg||C3-PTZ).

Conclusions:

• The developed Mg MBSBs exhibit remarkable stability, high voltage, and excellent cycling performance.
• These cost-effective batteries advance the application of Mg chemistry in organic flow batteries.
• The findings highlight the potential of MBSBs for next-generation energy storage solutions.