A solid-diffusion-free hydronium-ion battery with ultra-long cycle life over 8000 cycles

Hydronium-ion batteries possess the advantages of high conductivity, good economic benefits and environmental friendliness. However, inorganic solid electrode materials always suffer from structure collapse because of their insertion mechanism, while organic solid electrode materials always face dissolution challenges. Herein, a hydronium-ion battery with a soluble methylene blue (MB) anode and a MnO2@graphite felt cathode is proposed, involving a -C=N / -C-NH group transition at the anode and MnO2/Mn2+ at the cathode. The anode exhibits a maximum capacity of 1324 μAh cm−2 at 2 mA cm−2 and excellent cycle stability over 7500 cycles with a retention of 93 %. In- and ex-situ characterizations uncover that MB possesses a highly reversible interfacial redox reaction and excellent ring skeleton stability. By coupling with MnO2@graphite felt cathode, the MB//MnO2 batteries deliver an energy density of 198 μWh cm−2 and outstanding long cycle stability over 8000 cycles. Moreover, the batteries exhibit an excellent electrochemical performance at a low temperature of -20 °C with a capacity of 220 μAh cm−2 at 0.4 mA cm−2 and a remarkable capacity retention of 80 % over 5500 cycles. The achieved results provide a path for high-stability hydronium-ion batteries.