Construction Of Efficient Nickel Compound Catalysts For High-Performance Li-O₂ Batteries

Among the many secondary rechargeable batteries,Li-O2 batteries have attracted much attention owing to their extremely high theoretical energy density(3505 Wh kg-1 based on Li2O2 as discharge products),which is almost 10 times higher than that of Li-ion batteries.However,researches on them are still in the infancy stage,and a large number of challenges should be overcome before Li-O2 batteries can be used in practice,such as low specific capacities,poor cycling stability,high overpotentials,and poor safety.To address these issues,a great amount of efforts has been carried out in recent years,mainly including anode modification,construction of high-efficiency cathode catalysts,selective separators preparation,and exploration of stable electrolytes.It is widely accepted that efficient catalysts can not only accelerate the hysteretic kinetics of electrocatalytic reactions,but also promote the effective formation/decomposition of discharge products,thus improving the cycle stability.The exploration of ideal catalysts is thus crucial to improve the performance of Li-O2 batteries and promote their large-scale application in the future.Due to their abundant Ni2+/Ni3+redox pairs,nickel-based compounds have been extensively investigatged in secondary batteries and catalysis researches.During synthesis,it is easy to introduce defects to their structures,such as vacancies and dislocations,which can serve as catalytic active sites for oxygen reduction reaction/oxygen evolution reaction.In addition,they are extremely abundant and inexpensive in the earth’s crust,making them promising alternatives to precious metals as catalysts.However,the low electrical conductivites of nickel-based compounds results in low ion and charge transport rates,which greatly reduces the reaction kinetics.Therefore,based on the charging and discharging mechanism of Li-O2 batteries,this paper systematically studied the influence of different nickel-based catalytic materials on the electrochemical performance of Li-O2 batteries.(1)Hollow NiS2/NiSe2 homologous heterostructure cages(NiS2/NiSe2 HHS)were prepared by hydrothermal treatment and simultaneous selenization/sulfuration process,which were employed as cathode catalysts for Li-O2 batteries to study their electrochemical reaction mechanisms.The NiS2/NiSe2 HHS research shows the hollow cube structure with abundant pores on their surfaces,which effectively shorten the Li+and O2 transfer routes and allowed effective penetration of the electrolyte,providing enough space for storage of discharge products.In addition,lattice-matched NiS2 and NiSe2 with different Fermi energy levels combined together to achieve thermodynamic equilibrium,resulting in different charged zones with opposite charges and strong built-in electric fields at the heterogeneous interfaces.The NiS2/NiSe2 HHS cathodes show excellent catalytic activity,including high discharge/charge specific capacities(18408/18164 mAh g-1 at 100 mA g-1)and excellent cycling stability(more than 290 cycles at 500 mA g-1 with a fixed specific capacity of 600 mAh g-1).Density function theory calculations based on first-principles show that NiS2/NiSe2 HHS can significantly reduce the adsorption energy of oxygen-containing intermediates and effectively tune the formation route of Li2O2 via a surface/solution-mediated route.(2)Nitrogen-incorporated Ni5P4 electrocatalysts based on delocalized electronic engineering have been successfully prepared by the hydrothermal and annealing method.Nitrogen doping was applied to regulate the d-band centers of Ni5P4 nanoroses to achieve excellent oxygen reduction reaction/oxygen evolution reaction activities.The nitrogen-incorporated Ni5P4 nanoroses exhibit excellent catalytic performance with discharge/charge specific capacities up to 25103/24160 mAh g-1 at a current density of 100 mA g-1 and an excellent cycling performance of 237 cycles at an ultra-high current density of 3000 mA g-1.Further X-ray absorption spectroscopy and density function theory calculations results demonstrate that the enhanced performance can be attributed to a downward shift of the d-band center and accelerated electron transfer of Ni5P4 after binding to nitrogen,reducing the binding effect of catalyst and oxygen-containing intermediates and thus improving the performance of Li-O2 batteries.