Linear π-conjugated organic cathodes with dispersed redox-active units for high performance aqueous zinc-ion batteries

Organic molecules are promising electrode materials for aqueous zinc-ion batteries because of their intrinsic renewable nature and highly designable molecular architecture, but they still suffer from low capacity and unsatisfactory cycling stability due to the low conductivity, high solubility and thereby low utilization of active units. Herein, two types of linear pi-conjugated organic cathodes (PNZ-PTO and DANQ-PTO) are comparatively synthesized. The PNZ-PTO cathode with well-dispersed active units exhibits much more superior rate capability of 209 mAh g- 1 at 1 A g- 1, compared to that 58 mAh g- 1 for DANQ-PTO cathode with relatively concentrated active units. It is also remarkable to see that even at 2 A g- 1 the PNZ-PTO cathode can deliver excellent long-term cyclic stability with specific capacity of 147 mAh g- 1 and super-high retention of 96.5 % after 5000 cycles. The achievement can be attributed to the optimized PNZ-PTO structure with higher conductivity, lower solubility, and well-dispersed active units, promoting fast charge transfer and smooth ionic storage along with much higher active sites utilization of 80 % (only 66 % for DANQ-PTO cathode). Moreover, the charge storage mechanism based on Zn2+/H+ co-storage is also demonstrated by detailed investigations including various ex-situ characterizations, electrochemical tests, and DFT calculations.