Simultaneous acceleration of sulfur reduction and oxidation on bifunctional electrocatalytic electrodes for quasi-solid-state Zn–S batteries

The incomplete sulfur reduction and high ZnS re-oxidation energy barrier along with severe side reactions during the battery cycling compromise the practical application of Zn–S electrochemistry. Herein, a bifunctional electrocatalytic sulfur matrix that simultaneously accelerates the sulfur reduction and ZnS oxidation is proposed to realize a highly-efficient Zn–S cell. It is revealed that the N-heteroatom hotspots are more favorable for facilitating the conversion of S to ZnS while the CoO nanocrystal substantially lowers the ZnS activation energy barrier thereby suppressing the formation of disproportionation species (e.g., \({\rm{S}}{{\rm{O}}_4}^{2 - }\) $${\text{SO}}_4{}^{2-}$$ ) and accumulation of inactive ZnS. Accordingly, the CoO anchored on the N-doped carbon-supported sulfur cathode delivers a high Zn²⁺ storage capacity of 1,172 mAh g⁻¹ and outstanding cycling stability with a capacity retention of 71.6% after 500 cycles with a high average Coulombic efficiency of 97.8%. Simultaneously, the stable cycling of solid-state Zn–S pouch cells with an energy density of 585 \({\rm{Wh}}\,\,{\rm{k}}{{\rm{g}}^{ - 1}}_{{\rm{sulfur}}}\) $$\text{Wh}{\text{kg}}^{-1}{}_{\text{sulfur}}$$ is also demonstrated. Moreover, the postmortem analysis reveals that the degradation of Zn–S cells is mainly attributed to the limited reversibility of Zn anodes rather than the ZnS decomposition and/or accumulation. The approach to the bidirectional catalysis manipulated the sulfur redox provides a new perspective to realize the theoretical potentials of Zn–S cells.