Skeletal isomerization of dithiophenebenzo-4,5-dione enabling the development of an 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 properties that can be facilely tuned 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 into structure–property relationships is urgently needed. Skeletal isomerization provides a good platform for investigating structure–property relationships of organic functional materials, and 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 a carbonyl group at the α-position of the thiophene, exhibits a lower-lying LUMO level and more delocalized LUMO distribution than PBD-2, with a carbonyl group at the β-position of the 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. Moreover, a full cell is successfully assembled, revealing a stable discharge capacity of 105 mAh g⁻¹ during 400 cycles (1 A g⁻¹), indicating potential practical applications.