Triplet Sulfur Radical Pairs Stabilized through Hund’s Rule for Ultrafast Lithium–Sulfur Batteries

Sulfur radicals are highly reactive intermediates that can greatly accelerate reaction kinetics in lithium-sulfur batteries. However, the intrinsic instability restricts their applications. Herein, we reveal and validate the formation of ultrastable triplet sulfur radical pairs ([S_x·^- - S_x·^-], x = 2, 3, 4) by combining electron paramagnetic resonance and synchrotron radiations. These radical pairs are produced during the spontaneous decomposition of polysulfide molecules on ferrimagnetic surface, where the sulfur radicals adopt parallel spin alignment and pair into stable triplet states through Hund's Rule. These radicals enable exceptionally rapid sulfur conversion, delivering a 100-fold kinetic enhancement compared to the traditional polysulfide molecules. Using these triplet radical pairs, the lithium-sulfur battery achieves a remarkable discharge capacity of 728 mAh g^-1, even at an ultrahigh current rate of 8.0 C, with high sulfur loading and lean electrolyte. Significantly, this is the highest capacity under ultrafast charge-discharge rates reported to date.