Superior sulfur-doped carbon anodes for sodium-ion batteries through incorporation of onion-like carbon

Sodium ion batteries (SIB) are important components of a future energy system. Therefore, high-performance electrode materials for SIBs are in high demand. This study presents a sulfur-doped carbon ( SdC ) anode material, synthesized via the pyrolysis of polythiophene, without the use of elemental sulfur. The abstinence of elemental sulfur avoids polysulfide formation and shuttling, while the incorporated sulfur assures a high doping level. The anode was further optimized through the incorporation of onion-like carbon (OLC) and Super P65 carbon black (CB) as additives. The resulting OLC composite exhibited superior performance, including remarkable performance values like initial coulombic efficiency of 79.9 %, a maximum reversible capacity of 383 mAh/g at 0.05 A/g, and a retained capacity of 244 mAh/g after 500 cycles at 1 A/g. For sulfur rich polymer precursors, without adding additional elemental sulfur, these values are amongst the highest reported so far. The enhanced performance of the OLC composite is attributed to the improved sodium-ion accessibility by the porous onion-like carbon structure, which enables an increase of surface redox activity. These findings highlight the importance of carbon additives in optimizing the electrochemical behaviour and cycling stability of heteroatom-doped carbon anodes for sodium-ion battery applications, even with a simple polymer precursor without any templating or additional doping.