Visualization of Asymmetric Calcium-Sulfur Conversion Kinetics for High-Capacity Sulfur Cathodes in Calcium Metal Batteries.

Sulfur with sixteen-electron transfer per unit (S8) is a promising candidate for high-energy and sustainable calcium metal batteries. However, current Ca-S batteries (CSBs) suffer from poor capacity utilization and low Coulombic efficiencies, and origins of these limitations remain elusive. Herein, we visualized the Ca-S conversion dynamics using an operando optical microscopy platform. By extracting the calciation/decalciation reaction rates and polysulfide transport parameters, we discovered an apparent kinetic asymmetry, where the charging kinetics and polysulfide migration rates are substantially slower than those during discharge under identical conditions. Combined theoretical simulations and experimental measurements indicate that this asymmetry is associated with coupled interfacial barriers, including sluggish sulfur nucleation, polysulfide activation, and Ca2+ desolvation, which lead to insufficient polysulfide utilization and capacity degradation. To address this challenge, we demonstrate two strategies to mitigate the barrier via homogeneous growth on pre-setting sulfur seeds or regulating the interfacial barrier using a mediator. Consequently, the reversible capacity increases from 630 to 843 mAh g-1 with improved reversibility. This work elucidates the asymmetric Ca-S conversion kinetics and provides the mechanistic guidance for regulating sulfur electrochemistry in Ca metal batteries.