Manganese–nickel bimetallic oxide electrocatalyzing redox reactions of lithium polysulfides in lithium–sulfur batteries

Abstract

The inherent “solid–liquid–solid” multi-step reactions in lithium–sulfur batteries (LSBs) are accompanied by sluggish kinetics. In particular, redox reactions concerning short-chain lithium polysulfide (LiPS), Li₂S_x (x ≤ 4) ↔ lithium sulfide (Li₂S), are yet to be developed and understood. Since these reactions theoretically make over 70% contribution to the entire LSB capacitance, boosting their kinetics is essential for energy storage performance in practice. In this study, a MnO_x–NiO_x/C composite is synthesized via high-temperature calcination and cast on the positive side of a polypropylene (PP) separator. MnO_x–NiO_x/C is found to be a bi-directional electrocatalyst that accelerates Li₂S_x (x ≤ 4) redox reactions in charging/discharging cycles. During LSB discharging, MnO_x mainly strengthens the chemisorption of LiPS via S–Mn–O bonds, while Ni²⁺/Ni⁰ redox electric pairs in NiO_x catalytically promote LiPS conversion by promoting a thiosulfate-mediating path in parallel to LiPS reduction. Such an adsorption-catalysis synergistic effect between MnO_x and NiO_x intensifies Li₂S_x (x ≤ 4)-related reduction. During LSB charging, both MnO_x and NiO_x play the role of catalysts in the oxidation of Li₂S by optimizing the nucleation and decomposition of Li₂S (Li₂S₂ ↔ Li₂S). As a consequence, a LSB with a MnO_x–NiO_x/C modified PP separator gives a stable and attractive rate performance for long-term cycling (510 mA h g⁻¹ at 2.0C after 700 cycles). When the electrolyte dosage is reduced to E/S = 11 μL mg⁻¹, remarkable rate performance (630 mA h g⁻¹ at 3.0C) and superior cycling stability (620 mA h g⁻¹ after 400 cycles at 0.5C) are achieved.