posted on 2022-12-19, 15:03authored byXiaojuan Wen, Longfei Ren, Dayue Du, Yu Yan, Haoyang Xu, Ting Zeng, Guilei Tian, Xinxiang Wang, Sheng Liu, Chaozhu Shu
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,
t2g6eg0) 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.