"Innocent" Hexafluorophosphate Salts Induce Capacity Fade in Nonaqueous Redox Flow Batteries

Area of Science:

• Electrochemistry
• Materials Science
• Sustainable Energy

Background:

• Organic active materials offer a sustainable path for large-scale energy storage in redox flow batteries (RFBs).
• Nonaqueous organic RFB electrolytes face stability challenges due to degradation reactions causing capacity fade.
• Existing understanding attributes capacity fade to the conversion of redox-active organics into inactive products.

Purpose of the Study:

• To investigate the underlying mechanisms of capacity fade in nonaqueous organic redox flow batteries.
• To identify degradation pathways beyond the direct conversion of active organic materials.
• To explore strategies for enhancing the long-term stability of nonaqueous RFB electrolytes.

Main Methods:

• Detailed study of capacity fade using 1,2,4-benzotriazin-4-yl radical (1) as a model compound in symmetrical batteries.
• Analysis of degradation pathways involving supporting electrolyte salts.
• Comparison of electrolyte stability with different supporting anions, including PF6-.

Main Results:

• A novel capacity fade pathway was identified, involving acid-induced decomposition of the PF6- supporting salt anion in the posolyte.
• This decomposition follows a nonlinear, autocatalytic mechanism and is a universal degradation reaction in nonaqueous RFB posolyte.
• Acid crossover to the negolyte causes protonation of the active material, limiting capacity.
• Replacing PF6- with alternative anions dramatically improved electrolyte stability, enabling over 69 days of cycling with ≤0.1% daily fade rate.

Conclusions:

• Capacity fade in nonaqueous organic RFBs is significantly influenced by the supporting electrolyte salt, not just the active material.
• Acid-induced decomposition of PF6- is a critical degradation pathway that can be mitigated by anion substitution.
• This research provides a fundamental understanding for developing more durable and efficient nonaqueous redox flow batteries for sustainable energy storage.