It is shown that two commonly used performance metrics for charged redoxmers in flow batteries, viz. their cycling and calendar lifetimes, generally do not correlate with each other. It is also demonstrated that the cycling stability is extremely sensitive to experimental detail; unless the conditions are tightly controlled, this metric is unsuitable as a guide for materials development.
Significance and Impact
Extensive sets and kinetic modeling were used to characterize chemical stability of charged dialkoxyarene redoxmers in electrolyte bulk (calendar stability) and during cycling of a symmetric cell (cycling stability). While it is commonly believed that the two quantites are closely related, no such relation was found in our experiments.
- The cycling stability is shown to be defined by low-yield reactions yielding polymeric byproducts whereas the calendar stability is determined by the main reactions of charged redoxmers not leading to the polymers.
- Recommendations are suggested how to improve reliability of performance characterization so that it can be used for machine based materials discovery.