Metal valence state-regulated Li bond chemistry for efficient lithium–sulfur battery catalysis: A case study of cupric and cuprous oxides

Abstract

Valence state is identified as a key property of transition metal-based catalysts in conventional heterogeneous catalysis research. For a specific monometal element, however, the regulatory role of valence state has seldom explored in emerging energy catalytic applications such as rechargeable lithium–sulfur batteries suffering from sluggish sulfur cathode conversion kinetics. In this study, two monometal oxides with distinct valence states, cupric oxide (CuO) and cuprous oxide (Cu₂O), were investigated, revealing valence-state-dependent interactions between oxides and sulfur species, as well as the modulated sulfur reduction reaction (SRR) kinetics. In addition to the inherent Cu²⁺-enabled surface (poly)thiosulfate redox, divalent Cu²⁺ and monovalent Cu⁺ were found to steer the oxygen reactivity and so indirectly tune the lithium bond strength that dictates the surface chemisorption of lithium (poly)sulfides. The stronger interactions between CuO and sulfur species promoted SRR conversion kinetics, enabling enhanced lithium–sulfur battery performance under kinetically demanding conditions such as high-rate capability at 2 C with a moderate sulfur loading of 1.3 mg cm⁻² and cycling stability for over 110 cycles at a high sulfur loading of 4.8 mg cm⁻². This work is expected to expand the scope of metal-valence-state effect on heterogeneous catalysis and offer an unconventional “indirect” way to regulate lithium-bond chemistry for battery research.