Anion‐Regulated Sulfur Conversion in High‐Content Carbon Layer Confined Sulfur Cathode Maximizes Voltage and Rate Capability of K–S Batteries

Potassium–sulfur (K–S) batteries have attracted attention in large‐scale energy storage systems. Small‐molecule/covalent sulfur (SMCS) can help to avoid the shuttle effect of polysulfide ions via solid–solid sulfur conversion. However, the content of SMCS is relatively low (≤40%), and solid–solid reactions cause sluggish kinetics and low discharge potentials. Herein, SMCS is confined in turbo carbon layers with a content of ≈74.1 wt% via a C/S co‐deposition process. In the K–S battery assembled by using as‐fabricated SMCS@C as cathode and KFSI‐EC/DEC as an electrolyte, anion‐regulated two‐plateau solid‐state S conversion chemistry and a novel high discharge potential plateau at 2.5–2.0 V with a remarkable reversible capacity of 384 mAh g−1 at 3 A g−1 after 1000 cycles are found. The SMCS@C||K full cell showed energy and power density of 72.8 Wh kg−1 and 873.2 W kg−1, respectively, at 3 A g−1. Mechanism studies reveal that the enlarged carbon layer space enables the diffusion of K+‐FSI‐ ion pairs, and the coulombic attraction between them accelerates their diffusion in SMCS@C. In addition, FSI− regulates sulfur conversion in situ inside the carbon layers along a two‐plateau solid‐state reaction pathway, which lowers the free energy and weakens the S─S bond of intermediates, leading to faster and more efficient S conversion. Small‐molecule/covalent sulfur is confined in carbon layers (SMCS@C) with a record‐high SMCS content of 74.1 wt%. A novel FSI− regulated sulfur conversion chemistry is observed in K–S battery, leading to fast and more efficient S conversion. The SMCS@C//K full cells show energy/power densities of 72.8 Wh kg−1 and 873.2 W kg−1, respectively, at 3 A g−1.