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

Abstract

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 ≈74.1wt% 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 2.5-2.0V with a remarkable reversible capacity of 384mAhg-1 at 3Ag-1 after 1000 cycles are found. The SMCS@C||K full cell showed energy and power density of 72.8Whkg-1 and 873.2Wkg-1, respectively, at 3Ag-1. Mechanismstudiesrevealthat theenlarged 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 thecarbon 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.