Sulfur-Grafted Hollow Carbon Spheres for Potassium-Ion Battery Anodes

Sulfur-rich carbons are minimally explored for potassium-ion batteries (KIBs). Here, a large amount of S (38 wt%) is chemically incorporated into a carbon host, creating sulfur-grafted hollow carbon spheres (SHCS) for KIB anodes. The SHCS architecture provides a combination of nanoscale (≈40 nm) diffusion distances and CS chemical bonding to minimize cycling capacity decay and Coulombic efficiency (CE) loss. The SHCS exhibit a reversible capacity of 581 mAh g^-1 (at 0.025 A g^-1 ), which is the highest reversible capacity reported for any carbon-based KIB anode. Electrochemical analysis of S-free carbon spheres baseline demonstrates that both the carbon matrix and the sulfur species are highly electrochemically active. SHCS also show excellent rate capability, achieving 202, 160, and 110 mAh g^-1 at 1.5, 3, and 5 A g^-1 , respectively. The electrode maintains 93% of the capacity from the 5th to 1000th cycle at 3 A g^-1 , with steady-state CE being near 100%. Raman analysis indicates reversible breakage of CS and SS bonds upon potassiation to 0.01 V versus K/K⁺ . The galvanostatic intermittent titration technique (GITT) analysis provides voltage-dependent K⁺ diffusion coefficients that range from 10^-10 to 10^-12 cm² s^-1 upon potassiation and depotassiation, with approximately five times higher coefficient for the former.