In Situ Mechanistic Insights into the Divergent Electrochemical Behaviors of Li, Na, and K in Metal Sulfur Batteries

Metal–sulfur batteries are promising candidates for next-generation energy storage, yet the origins of their distinct reaction behaviors remain elusive. Here, we employ a noncatalytic vapor-grown carbon fibers/sulfur (VGCF/S) interface to regulate sulfur electrochemistry. In situ TEM reveals highly reversible two-step redox reactions and rapid reaction-front propagation in Li–S nanobatteries. Coin-cell measurements further show reversible cycling and rate capability for Li–S under specific testing conditions. DFT calculations in idealized graphitic and sulfur–carbon environments provide a qualitative comparison of alkali-species-dependent migration. The observed divergence may reflect coupled contributions from alkali-species transport, interfacial processes, electrolyte-dependent sulfur-redox pathways, and product evolution. This work highlights noncatalytic interfaces as a platform for probing sulfur electrochemistry.