In-situ TEM Study Of Lithium-related Solid State Electrochemical Processes At Nanoscale

As an important branch of transmission electron microscopy(TEM),in-situ TEM has drawn tremendous attention from various fields ranging from materials science to chemistry and biology.In-situ TEM offers a unique capability to image the atomic structure of materials in real time under various external stimuli at high tempo-spatial resolutions while simultaneously measuring relevant properties.A variety of in-situ TEM holders have been developed to enable imaging and measurements under applied heat,stress,optical excitation,and magnetic or electric fields.Among these diverse kinds of in-situ TEM techniques,in-situ electrical probing TEM is the best choice to probe the solid-state nanoscale electrochemical reactions.Benefited from the advanced piezoelectricity control technique,we are able to realize precise control of the electrical probe,and relocate the target areas with the guidance of TEM imaging at the same time.In recent years,in-situ electrical probing TEM has achieved great progesses in the studies of electrochemical and solid-state inoics processes,enriching our fundamental understanding on electrochemical dynamic processes and microscopyic mechanism.The research project in this dissertation is based on in-situ electrical probing TEM.By taking advantage of the ultrahigh vacuum condition in TEM chamber,we systematically investigated several specific lithium-related electrochemical processes at nanoscale,including the conversion-type electrochemical reaction of Cu O nanowires,electrochemical solid-state amorphization in the immiscible Cu-Li system mediated by nanoscale size effect and in-situ electrochemical synthesis of novel two dimensional nanosheets of alkali metals.The main research and results are as follows:1.In-situ and real-time studies of the nanoscale electrochemistry of transition metal oxides is of both fundamental interest and practical relevance to applications involving electronics and ionics.Here the dynamic electrochemical lithiation process of Cu O nanowires(NWs)was investigated by in-situ TEM platform.We show that there exists an anomalous lithiation kinetics of Cu O NWs that is mediated by the specific sizes of NWs.That is,the lithiation rates of longer NWs turn out to be larger than those of the short ones,and more interestingly,these two ranges of NWs exhibit different lithiation kinetics relationships.Through systematic in-situ TEM studies,we elucidate the underlying mechanism that accounts for the observed anomalous electrochemical kinetics.For longer nanowires,the transport rate of electrons within the circuit is comparative with that of lithium ions,thus the reaction is Cu O Lie ? Cu Li O.While for shorter nanowires,due to the much faster transport rate of electrons,lithium ions are reduced to form metallic lithium before arriving at the interface of unreacted Cu O,and then lithium atoms diffuse to the interface of unreacted Cu O to react with Cu O.2.As a typical immiscible binary system,copper(Cu)and lithium(Li)show no alloying and chemical intermixing under normal circumstances.Here we show that,when decreasing Cu nanoparticle sizes into ultrasmall range,the nanoscale size effect can play a subtle yet critical role in mediating the chemical activity of Cu and therefore its miscibility with Li,such that the electrochemical alloying and solid-state amorphization will occur in such an immiscible system.This unusual observation was accomplished by performing in-situ studies of the electrochemical lithiation processes of individual Cu O nanowires inside a TEM.Upon lithiation,Cu O nanowires are first electrochemically reduced to form discrete ultrasmall Cu nanocrystals that,unexpectedly,can in turn undergo further electrochemical lithiation to form amorphous Cu Lix nanoalloys.Real-time TEM imaging unveils that there is a critical grain size(ca.6 nm),below which the nanocrystalline Cu particles can be continuously lithiated and amorphized.The possibility that the observed solid-state amorphization of Cu-Li might be induced by electron beam irradiation effect can be explicitly ruled out;on the contrary,it was found that electron beam irradiation will lead to the dealloying of as-formed amorphous Cu Lix nanoalloys.3.Thickness-controlled synthesis of two-dimensional(2D)non-layered materials is of scientific significance because non-layer structured 2D materials may possess intriguing properties and advanced functions that cannot be achieved for their counterparts in other dimensionalities.Here we report for the first time the in-situ growth of ultrathin 2D Li and Na nanosheets by taking advantage of an in-situ electrochemically-driven experimental framework inside TEM.Real-time observations revealed with unprecedented details the dynamic growth of 2D Li and Na nanosheets with diverse shape evolution while maintaining 2D sheet-like structure consistently.DFT calculation reveals that the stronger interaction of O2 on {111} planes prevents growth along [111] direction and further promotes the final formation of 2D alkali metal nanostructures.Cathodoluminescence spectroscopy has been performed on freshly formed 2D Li nanosheet in a TEM setup to detect the plasmon eigenmodes.Peaks appearing in the visible range are shown to arrive from excitation of out-of-plane eigenmodes by the electron beam.The discovery confers a unique method to electrochemically synthesize 2D alkali metals.