Skeleton Isomerization of Dithiophenebenzo-4,5-Dione Enabling Efficient n-Type Cathode Material for Sodium-Ion Batteries

Abstract

Organic n-type electrodes are promising in sodium ion batteries due to their high theoretical specific capacity, and the facilely tuned properties by structure modifications. However, the currently reported organic n-type electrodes encounter problems such as low voltage platform and poor cycling stability, and further investigation on structure-property relationships is still in urgent need. Skeleton isomerization provides a good platform for investigating structure-property relationship of organic functional materials, which has been rarely studied for electrode materials. Herein, new n-type polymers based on dithiophenebenzo-4,5-dione are developed as cathode materials. PBD-1 with carbonyl at the α-position of thiophene exhibits a lower-lying LUMO level and more delocalized LUMO distribution than PBD-2 with carbonyl at the β-position of thiophene, which is conducive to Na+ adsorption and electron transport during sodium storage. At a current density of 0.5C, the initial specific capacity of PBD-1 is 176 mAh g-1, with the capacity retention rate as high as 96.6% after 400 cycles. Notably, the PBD-1 electrode shows a high median voltage of 2.3 V, which is one of the highest values for n-type cathode materials. Moreover, full-cell is successfully assembled, revealing a stable discharge capacity of 105 mAh g-1 during 400 cycles (1 A g-1), manifesting the potential practical applications.