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 KPF6 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 similar to 106 mA h g(-1) for 3200 cycles at 500 mA g(-1). Using the reduced state (K4TP) of potassium terephthalate ((KTP)-T-2) as the organic anode, the resulting K4TP II AQ26OK organic potassium ion batteries can display a highly stable average discharge capacity of 135 mA h g(cathode)(-1) over 250 cycles at 100 mA g(-1) and similar to 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.