Thiophene-sulfur doping in nitrogen-rich porous carbon enabling high-ICE/rate anode materials for potassium-ion storage

Sulfur doping in carbon materials is generally considered to enhance potassium storage capacity and reaction kinetics. However, the contribution of sulfur doping to initial coulombic efficiency (ICE) is still lacking to date. Additionally, previously reported sulfur-doped carbons usually contain multiple sulfur configurations (such as thiophene-type sulfur, C-SOx-C and C-SH), which hinders the investigation of the role of thiophene-type sulfur in potassium ion storage. In this work, we have successfully prepared thiophene-sulfur-doped nitrogen-rich porous carbons with different sulfur contents via a simple high-temperature sulfurization strategy assisted by pre-carbonization. The K half-cell tests show that thiophene-sulfur doping can significantly elevate the ICE of the samples and that there is a positive correlation between the ICE of the samples and thiophene-sulfur doping levels. The higher ICE (60% vs. 42%) of the optimized thiophene-sulfur-doped nitrogen-rich porous carbon (S1-NC) than the sulfur-free nitrogen-rich porous carbon (NC) derives from the increased reversibility of the electrode material, which is demonstrated by the ex situ XPS and in situ Raman tests. Moreover, the optimized S1-NC delivers nearly twice the specific capacity (424.0 mA h g-1 at 0.1 A g-1) over NC, ultra-fast rate performance (197.2 mA h g-1 at 10 A g-1) and excellent cycling stability (206 mA h g-1 with 81.7% retention after 2000 cycles at 2 A g-1), which can be attributed to numerous additional active sites and enhanced potassium storage kinetics (including elevated diffusion coefficients and ionic conductivity, as well as a high percentage of capacitive behavior) inspired by the thiophene-sulfur doping. This work offers an insight into the contribution of thiophene-sulfur doping to the ICE/specific capacity/potassium storage kinetics of carbon anode materials. Sulfur doping in carbon materials is generally considered to enhance initial coulombic efficiency, potassium storage capacity and reaction kinetics.