Balance of sulfur doping content and conductivity of hard carbon anode for high-performance K-ion storage

Sulfur-doped hard carbon (SC) with high capacity has been considered as a promising anode for potassium ion batteries (PIBs). However, due to the uncontrollable pyrolysis, the sulfur doping content and conductive carbon network of SC exhibit an intrinsic trade-off, resulting in poor rate performance and cycle stability. To address this issue, we chose highly conjugated polythiophene (Pth) as precursor for the easy formation of sp2 carbon skeleton with high sulfur doping. With increasing pyrolysis temperature, the structural evolution of Pth such as successive desulfuration, dehydrogenation and cyclization is systematically verified. Based on the balanced control of pyrolysis degree, sulfur-doped hard carbon at 700 °C (SC-700) has a high sulfur content (7.59 at.%) and high conductivity (56.5 S m−1), affording a high reversible capacity (442 mAh g−1 at 0.05 A g−1), excellent rate capability (162 mAh g−1 at 10 A g−1) and exceptional cycling stability (87% retention at 1 A g−1 after 1000 cycles). Ex-situ Raman and X-ray photoelectron spectroscopies demonstrate that the balance between sulfur doping and conductivity not only improve the faraday capacitance, but also promote K+ intercalation kinetics. The strategy can provide novel enlightenment for designing heteroatom doped carbon in energy storage and conversion.