Activating lithium sulfide cathodes through localized interfacial proton exchange for high-efficiency energy storage

Lithium–sulfur batteries are compelling candidates for next generation energy storage, but the practical use of lithium sulfide (Li 2 S) cathodes is hindered by the exceptionally high voltage required for initial activation. Here, we demonstrate a proton-mediated electronic-structure engineering strategy to lower the activation barrier. By inducing a controllable solid-state proton-exchange reaction between Li 2 S and ammonium bifluoride (NH 4 HF 2 ), protons interact with Li 2 S at localized interfacial regions without compromising its structural integrity. Spectroscopic measurements, in situ electrochemical analysis and ab initio molecular dynamics reveal that proton incorporation redistributes electron density around sulfur, weakens Li-S interactions and lowers the activation barrier for delithiation, reducing the initial activation voltage to 2.38 V vs. Li/Li + . Proton exchange also alters subsequent sulfur-conversion chemistry by decreasing the population of soluble polysulfide intermediates, thereby suppressing shuttle-related side reactions despite a modest kinetic penalty in later cycles. These findings establish controlled, localized proton exchange as an effective strategy for activating Li 2 S cathodes and offer a general framework for regulating metal-sulfide electrode reactions in high-energy electrochemical storage systems.