Insoluble polyanionic anthraquinones with two strong ionic O-K bonds as stable organic cathodes for pure organic K-ion batteries

A new organic cathode namely potassium 2,6-dihydroxyanthraquinone (AQ26OK, theoretical capacity ( C T ) = 169 mA h g −1 ) is synthesized and fully characterized for K-ion batteries. AQ26OK is called polyanionic organic cathode because it has a polyanionic organic skeleton (−2 valent) and two strong ionic K-O bonds. Consequently, the polyanionic AQ26OK is hardly soluble into most organic liquid electrolytes. In half cells (0.3–3.4 V vs. K + /K) using 1 mol L −1 KPF 6 in dimethoxyethane, AQ26OK delivers a highly stable specific capacity of 201 mA h g −1 @50 mA g −1 over 450 cycles (4-month test) and realizes ~106 mA h g −1 for 3200 cycles at 500 mA g −1 . Using the reduced state (K 4 TP) of potassium terephthalate (K 2 TP) as the organic anode, the resulting K 4 TP II AQ26OK organic potassium ion batteries can display a highly stable average discharge capacity of 135 mA h g −1 cathode over 250 cycles at 100 mA g −1 and ~47 mA h g −1 for 1000 cycles at 500 mA g −1 during the working voltage of 0.01–3.1 V. To the best of our knowledge, AQ26OK is among the best stable cathodes reported for K ion batteries.