MOF-Derived CoO_x–CeO_2−y/C Heterojunction Synergistically Promotes the Deposition and Decomposition of Li₂S in Lithium–Sulfur Batteries

Abstract

The sluggish bidirectional conversion rate between Li₂S_n (2 ≤  n ≤ 4) and Li₂S, coupled with the uncontrolled deposition of Li₂S, significantly impedes the realization of high-performance lithium–sulfur batteries (LSBs). In this study, a metal–organic framework was employed as a precursor for the synthesis of a CoO_x–CeO_2−y/C (0 <  x < 3/2, 0 <  y < 1/2) heterojunction via pyrolysis, which was subsequently introduced onto the cathode side of the polypropylene (PP) separator in LSBs. The modification of CoO_x–CeO_2−y/C enhances the kinetics of converting of Li₂S_n to Li₂S during the discharge process. The Tafel slope for the Li₂S deposition reaction is reduced to 52.1 mV/dec, representing a 56.6% decrease compared to LSBs with bare PP separator. Conversely, during the charging process, the modification lowers the energy barrier for the Li₂S decomposition reaction, with the activation energy reduced to 6.12 kJ/mol, indicating a 70.3% decrease relative to LSBs with PP separator only. Consequently, more Li₂S is promoted to undergo decomposition. The CoO_x–CeO_2−y/C heterojunction facilitates uniform deposition of Li₂S, featuring fine particles and a uniform distribution, after brief potentiostatic charging for decomposition, thereby effectively mitigating the deactivation of sulfur species. Thanks to the enhanced bidirectional conversion of lithium polysulfides (LiPS) facilitated by the CoO_x–CeO_2−y/C modification layer, the (−)Li|CoO_x–CeO_2−y/C@PP|S(+) coin cell maintains a Coulombic efficiency of 90.4% after 500 cycles at a current density of 1 C, exhibiting a low capacity-decay rate of only 0.081% per cycle, thereby demonstrating excellent long-cycle stability.

Keywords

Separator modification / Heterojunction / Synergistic effect / Lithium–sulfur batteries

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