High-Content Carbon Layer Confined Small-Molecule/Covalent Sulfur Cathode Enables Long-Life Calcium-Sulfur Batteries in Hybrid-Ion Electrolyte

Calcium-sulfur (Ca-S) battery is a promising candidate for large-scale energy storage system. However, the development of long-life room-temperature Ca-S batteries is hindered by the lack of effective sulfur cathode and the incompatibility between electrodes and electrolytes, leading to irreversible sulfur conversion, inefficient calcium plating/stripping and improper cathode/anode-electrolyte interfaces (CEI/SEI). Herein, a turbo carbon layer confined small-molecule/covalent sulfur cathode (S@C NS) with a record-high content of 65.7% and enlarged carbon layer space is prepared via a C/S co-deposition process. By using such a cathode and optimizing interfacial electrochemistry in LiPF6/Ca(BF4)2-based hybrid electrolyte, a record-breaking long-life room-temperature Ca-S battery is reported. The capacity reaches 824.6 mAh g-1 at 100 mA g-1 with a retention of 47.9% after 145 cycles, and 462.1 mAh g-1 at even 500 mA g-1. Mechanism studies demonstrated that a Li/Ca-based hybrid CEI with porous microstructure is found to facilitate the transfer kinetics of Ca2+ across the CEI; Turbo carbon layers with large inter-layer space and their strong confinement on small-molecule sulfur enabled reversible solid-state sulfur conversion in situ inside the cathode; An effective Li/Ca-based hybrid SEI formed on anode, allowing reversible Ca plating/stripping with low deposition potential and small overpotential. A carbon layer confined sulfur composite (S@C NS) is synthesized with high sulfur content and large carbon layer space. Ca-S batteries utilizing S@C NS and a unique hybrid electrolyte showcase reversible solid-solid sulfur conversion and hybrid CEI with porous microstructure, avoiding shuttle effect and boosting Ca2+ transfer kinetics, which thus leads to a record-long life and high reversible capacity.image