In Situ Transmission Electron Microscopy Study On The Electrochemical Reaction Mechanism Of Nickel-based Sulfides

As one of the advanced,effective and safe energy storage technologies,lithium-ion batteries have been widely used in portable devices and electric automobile of modern society right now,which greatly facilitates the human lives.Since the commercialization of lithium-ion batteries in the 1990s,it was grabbed much attention on the exploration and research of advanced electrode materials for lithium-ion batteries,and numerous anode materials with high specific capacity,high energy density and good cycling stability have been successfully developed,including graphene anode with insertion/extraction reaction type,silicon anode with alloying reaction type.Transition metal sulfides show great potential as reversible electrode materials for rechargeable lithium-ion batteries due to their simple preparation,diverse microstructures,and good compatibility with other materials in composite.Especially,nickel-based active materials are widely used in both cathode and anode materials for lithium-ion batteries because of their abundant reserves,low price,high theoretical capacity and environmental friendliness.Pure nickel-based active materials usually can not satisfy the performance require for rechargeable batteries when used as anode materials.After forming composite with carbon substrate,both reversible capacity and cycling performance of the battery can be obviously improved.However,it is still enclusive for understanding of the electrochemical reaction and performance optimization mechanism of nickel-based anodes.In this thesis,one all-solid-state nanobattery system was successfully constructed in transmission electron microscopy based on the high spatial resolution of advanced transmission electron microscopy.Firstly,and the reliability and availability of the nanobattery system was tested by using york-shell silicon as high-capacity anodes for lithium-ion batteries,observing the real-time morphological and phase evolution in dynamic electrochemical reaction process.The results show that both the core and the shell were expanded upon lithiation and were shrinkage after full delithiation,however,it could not recover to the initial size after cycling;the insertion and precipitation of Li₂O on the surface of Si O_xhollow sphere indicate that it can be used as the container for storage lithium.Additionally,the electrical properties and mechanical stability of Si O_xanodes with the proceeding of lithiation changed largely,accompanying with the transition from insulator to conductor.All results above illustrate in-situ TEM is one of key methods for deeply study the dynamic electrochemical reaction process.Based on this nanobattery system,we firstly investigated the electrochemical reaction mechanism of nickel sulfide nanoparticles used as the anode of lithium-ion batteries.It was irreversible of the reaction in the first cycle by direct observation and analysis of morphology,structure and phase evolution of single nickel-based sulfide nanoparticles with different stoichiometric ratios during dynamic charging and discharging.Single crystal nickel-based sulfide（Ni S/Ni S₂）firstly transits to polycrystalline Ni₃S₂phase,and then,the corresponding Ni₃S₂transforms to polycrystalline Li₂S and Ni phases after full lithiation.Upon delithiation,the Ni polycrystals react with Li₂S to form Ni₃S₂phase,and the reversible phase transition occurs between Ni₃S₂and Ni and Li₂S in the subsequent cycles.The volume of nickel-based sulfides has expansion upon lithiation,we studied the key factor for determined the structural stability in cycles,revealing that both carbon substrate and the initial size of nanoparticles mainly influence its stability.While nickel sulfide without carbon substrate cannot maintain the structural integrity in the first lithiation process,however,after forming the composite with graphene,the morphology can return to its initial status upon full delithiation,which explains the performance optimization mechanism of carbon addition in nickel-based anodes.Moreover,a comparative study of the morphological stability of nickel sulfide particles with size range of<50 nm,100～150 nm,150～200 nm,200～250nm,>300 nm,etc.The dynamic results display that the nanoparticles are completely pulverized when the original size exceeds 180 nm,even carbon substrate exists,leading to the capacity loss and cycling performance decay.That’s because the constrain of carbon substrate for the loading nanoparticles is not enough to resist the internal stress change induced by the lithiation.Based on the above study and analysis for the nickel-based anodes in the dynamic reaction process,we found that the single crystal gradually collapse with the proceeding of phase transformation with forming of porous polycrystalline structure;while in the delithiation process,“rebuilding”occurs in porous structure as lithium extraction,accompanying with aggregation and growth of small grains,and the disappearance of pores meanwhile.Therefore,the“rebuilding”is mainly determined by the substrate,however,the initial size of nanoparticles show less effect on this behavior.Moreover,we have found that the“rebuilding”results from formation of Ni₃S₂phase in delithiation process.In this thesis,the nanobattery system was successfully developed,it provides experimental guidance for the structural design and performance optimization of high-capacity electrode materials,with deeply study the electrochemical reaction mechanism and the optimization of carbon substrate.