Spin-State Regulation of Nickel Cobalt Spinel toward Enhancing the Electron Transfer Process of Oxygen Redox Reactions in Lithium-Oxygen Batteries

The development of a high-energy-density lithium- oxygen (Li-O2) battery is mainly determined by highly efficient electrocatalysts with excellent activity and stability, facilitating reversible oxygen redox reactions. Engineering the electron spin state provides a novel strategy to enhance the electrocatalytic activity of electrode materials. In this work, sulfur vacancy-enriched spinel NiCo2S4 (Vs-NiCo2S4) with high spin polarization is designed as an effective electrocatalyst for high-performance Li- O2 batteries. Subtle lattice distortion is induced by sulfur vacancy in spinel Vs-NiCo2S4, which strongly contributes to the formation of high spin states of Co3+ (HS, t2g4eg2) from the low spin states of Co3+ (LS, t2g6eg 0) at active octahedral sites. The electron transitions of Co3+ from low to high spin enable the increase of the spin polarization and produce abundant unpaired electrons in the 3d orbital of Co3+, enhancing the adsorption of oxygen intermediates and boosting the electron transfer process of oxygen redox reactions. Density functional theory calculations indicate that the spin magnetic moment of Co3+ in Vs-NiCo2S4 is raised in comparison to that in NiCo2S4, which improves the catalytic activity of the material via lowering the energy barrier for electron transfer. Experimentally, Vs-NiCo2S4-based Li-O2 batteries exhibit a large specific capacity of 8707.0 mAh g-1 and long cycling life of 487 h. Furthermore, in situ differential electrochemical mass spectrometry results show that the ratio of electron to oxygen during oxygen redox reactions is close to 2, demonstrating the favorable formation and decomposition of Li2O2 on Vs-NiCo2S4 in a Li-O2 battery. This work presents a powerful strategy to rationally design efficient electrocatalysts via spin engineering to boost oxygen redox reaction kinetics in Li-O2 batteries.