In Situ TEM Study Of The Nanoencapsulation Of Alkali Metals With Amorphous Carbon Nanotubes

The theoretical capacity of lithium metal anodes is as high as 3860 mA h g-1,which is about ten times the capacity of graphite anodes in traditional lithium-ion batteries.Lithium metal batteries constructed with this can meet the urgent needs of today's society for high-energy-density secondary batteries,so it has led to a new round of research on the construction of safe,stable and high-performance lithium metal anodes.The main obstacles in the commercialization of lithium metal anodes include lithium dendrite growth during cycling,unstable electrode/electrolyte interface,and huge volume expansion.This thesis attempts to use amorphous carbon nanotubes(aCNTs)to encapsulate lithium metal to solve the above problems lithium metal anodes are facing.In order to better analyze its working mechanism,we use in-situ TEM techniques to conduct real-time observations on the structural and compositional changes of the deposition/stripping processes of lithium metal in aCNTs.This is critical to understand the mechanism of the Li encapsulation processes,and can provide reliable data for the design of more efficient carbon-based metal anodes.Meanwhile,we also extend the concept of nano-encapsulation to the in-situ investigation of other alkali metal anodes,such as sodium and potassium metal anodes.The main results are summarized as follows:We first prepared amorphous carbon tubes Au@aCNTs embedded with gold nanoparticles as a host to encapsulate lithium metal,and studied the Au-guided Li deposition/stripping kinetic behaviors in its one-dimensional confined space by in-situ TEM.In-situ electron diffraction revealed a two-step reversible phase transition process of Au nanoparticles during lithiation and delithiation.It was found that Li3Au phase played a practical role in guiding lithium nucleation during the lithium deposition processes induced by Au nanoparticles.Combined with in-situ TEM observation,we proposed a "front growth" model to explain the growth kinetics of lithium metal in confined environment,that is,lithium ions transport along the tube wall and deposited at the growth front of the lithium metal.The stripping process of lithium metal also proceeds in a similar way.We also demonstrated for the first time that,stable encapsulation of sodium metal can also be achieved with the use of Au nanoparticle inside aCNTs.Furthermore,our subsequent researches show that even without the use of heterogeneous seeds,some aCNTs are capable of encapsulating lithium metal as well.Through control experiments,we found that the aCNTs enabling Li encapsulation had some structural and performance characteristics in common:(1)low carbonization temperature during the preparation process(optimum range:500-600? and high amorphization degree of the tube wall;(2)decent electronic and ionic conductivities;(3)the total element content of nitrogen and oxygen exceeding 10%,leading to its enhanced lithiophilicity due to the synergistic effect;(4)electrochemically-stable structure allowing for long-term cycling without lithium-induced embrittlement.The Li encapsulation process inside an aCNT typically includes the following three steps:(1)Li+ transport along the tube wall,followed by the lithiation of the carbon shell and Li filling in its nanopores;(2)after full lithiation,due to the synergistic lithiophilic effect of N and O functional groups on the inner wall,Li+is induced to nucleate inside the aCNT cavity;(3)Li metal grows as a single crystal from the nucleation site as the Li+transport continues,until the whole tube is filled up(lithium metal can also nucleate at multiple sites on the inner wall,and eventually form polycrystals inside the tube).In addition,in the in-situ TEM experiments of Li encapsulation,the amorphous carbon shell helps reduce the irradiation and thermal damage from the electron beam and acts as a protective layer for the encapsulated lithium metal.Therefore,without the use of cryo-electron microscopy,we can also capture high-resolution TEM lattice images of single-crystal lithium metal at room temperature.The above results are of much significance for understanding the alkali metal deposition guided by heterogeneous particles,as well as the dendrite growth behaviors in other spatially-confined environments(such as at the grain boundaries of solid electrolytes).This nano-encapsulation technique enables the full use of the internal space of many nanotube hosts thereby further greatly increasing their storage capacity,which would be helpful for the design of high-performance carbon-based alkali metal anodes.