Study On The Design Of Nanostructures And The Properties Of Lithium/Sodium Storage Of The Ni₃S₂ Material

Nanostructured nickel sulfides have been widely studied in photovoltaic cells,hydrogen evolution,supercapacitors,rechargeable lithium ion batteries?LIBs?,and rechargeable sodium ion batteries?SIBs?due to the various composition of nickel sulfides with different atomic ratios.Among them,Ni3S2 has a room temperature resistivity of approximately 1.2×10^-4?·cm,this effective metallic conductor can transport electrons better than other nickel sulfides.Therefore,Ni₃S₂ demonstrates a promising potential as an electroactive material for high-performance LIBs and SIBs.However,similar to other transition metal sulfides,growing parts of the Ni₃S₂ electrode will lose their electrical contact during the electrochemical reaction processes caused by pulverization,resulting in decreased utilization of active materials.On the other hand,nanostructured Ni₃S₂materials normally lead to the formation of a large amount of solid electrolyte interphase?SEI?between the electrode surface and electrolyte due to the high surface area,resulting in bad coulombic efficiency and poor cyclic stability.Thus,in view of the above problems,the main concentration of this paper is the design and the modification of the electrode structure of Ni₃S₂/Ni where Ni₃S₂ materials are directly grown on the nickel foam skeleton.The influences of acids on the surface of nickel foam and its effect on the growth of nickel sulfide are studied.Treated by sulfuric acid,oxalic acid,and phosphoric acid,three Ni foams are fabricated with different surface states containing different surface roughness and different surface layers with nickel salts.By controlling the reaction conditions,some Ni₃S₂ morphological structures with nanosheets,clustered nanosheets,nanoparticles and nanorods are designed on these preprocessed Ni foams.The influences of the morphology on the performance evolution for sodium-ion batteries were studied in detail.As a result,it was found that the initial drop,gradual increase,convex type decline and concave type decline in the performance evolutions were mainly controlled by R_ct,R_sf,pulverization,and internal stress,respectively.More importantly,our results indicated that the clustered network-like structure was beneficial for the cycling and rate performance,while the rod-like structure was suitable for the cyclic stability of these electrodes.A directly reduced graphene oxide?RGO?decorated anode electrode was designed and tested for SIBs,in which uniform RGO coating onto the Ni₃S₂/Ni electrode was realized using facile hydrothermal reactions.The results indicate that the RGO/Ni3S2/Ni electrode delivers a high reversible specific capacity of 448.6 mAh/g,high capacity retention of 96.5%after 100 cycles,and excellent rate capability of 263.1 mAh/g at 800mA/g.Compared with the Ni₃S₂/Ni electrode,the improved performance of the RGO/Ni₃S₂/Ni electrode benefits from RGO-promoted displacement reaction of Ni₃S₂with sodium.DFT calculations reveal that the RGO layer can significantly improve the electron mobility of the RGO/Ni₃S₂+Na structure,and the hybrid interaction between the extraneous p orbits of C and indigenous p and d orbits of Ni,as well as p orbits of S is the major reason for why RGO can improve the electrical transport properties.Ultrathin LiAlO₂ and Al₂O₃ layer coated Ni₃S₂/Ni have been synthesized based on atomic layer deposition?ALD?as anode materials for lithium ion batteries?LIBs?.X-ray photoelectron spectroscopy?XPS?reveals the formation of the Ni-O-Al bond.The cells with the ALD layers show improved cycle performance as compared with those of the noncoated Ni₃S₂/Ni.Especially,the LiAlO₂ coated Ni₃S₂/Ni demonstrates the excellent cyclability with a capacity retention of 98.8%after 100 cycles at 50 mA/g.This improvement is attributable to LiAlO₂ which works as a stable SEI film with fast diffussion of Li,suppresses the oxidation of solvents,and enhances the structural stability of the active materials during cycling.