Quadruply Fused Aromatic Heterocycles toward 4 V‐Class Robust Organic Cathode‐Active Materials

Abstract

Quadruply fused aromatic heterocycles are examined as 4 V‐class organic cathode‐active materials. A newly synthesized dimethylfluoflavin‐substituted polymer displays reversible charge/discharge at high potentials of 3.5 and 4.1 V (vs. Li/Li+) as a cathode‐active material for organic secondary batteries. The robust redox‐active heterocycles enable the long cycle‐life (>1000) while maintaining the high potential over 4 V. The linear polymer backbone and radical spin configuration in the heterocycles are the keys to enhancing the voltages and the cycle life.

Thianthrene polymers as 4 V-class organic mediators for redox targeting reaction with LiMn2O4 in flow batteries

“…An oxidation mediator to LiMn 2 O 4 should have a potential over 4 V. However, few organic compounds exhibit reversible redox reactions in the region because degradation often occurs by reacting with electrolytes or by themselves 20 . This article examined thianthrene derivatives, offering redox at over 4 V 26,27,[32][33][34] .…”

“…The expected voltage of the cell would be only 1.6 V, which is much smaller than conventional lithium-ion batteries (> 3 V). The low voltage comes from the few candidates of redox mediators working at higher (> > 3.4 V) or lower (< < 1.8 V) potentials, whereas conventional inorganic materials offer wide variations (e.g., graphite: 0.1 V, LiCoO 2 : 3.7 V, and LiMn 2 O 4 : around 4 V) 7 , 20 , 21 .…”

“…Reported potentials of most redox-active molecules have been limited up to 3.8 V 7 , 20 , 21 . The values were less than 3 V for most quinones 22 , 3.4 V for ferrocene 13 , 3.6 V for TEMPO 4 , around 3.7 V for 2,2,5,5-tetramethylpyrrolidine 1-oxyl (PROXYL) 21 , and ca.…”

“…Introduction of electron-withdrawing groups (e.g., SO 2 ) to the redox units could boost potentials over 4 V, whereas stability is sacrificed as a tradeoff 25 . Aromatic heterocycles, such as thianthrene (4.1 V) 26,27 , phenoxazine (3.7 V) 28 , and fluoflavin (3.7 V) 20,29 , are relatively new-class redox units showing high potentials. Still, they have not joined the mainstream of active materials for flow systems, certainly because of poorer solubility than the aliphatic species (i.e., due to π − π stacking) and insufficient data for cyclability 20,[26][27][28][29] .…”

Redox: Organic Robust Radicals and Their Polymers for Energy Conversion/Storage Devices

“…Aromatic heterocycles and conjugated polymers can also be classified as Class I-a in Figure because their radical cations are often persistent or robust. Phenothiazine, − phenazine, − phenoxathiin, phenoxazine, thianthrene, − carbazole, , and related heterocycles − offer bistable redox reactions and have attracted attention as organic electrode active materials. The radical cations can also sometimes be isolated .…”