In-situ Electron Microscopy Study Of Sodium Ion Storage And Transport In Carbon Nanofiber Anodes

As a rapidly developing technique,in-situ electron microscopy plays an important role in the study of micro and nano scale materials due to its unique capability of direct linking,the properties and the microstructures of materials,thus providing a powerful tool for deeper understanding of the mechanism of energy storage and conversion devices.Sodium ion batteries have attracted great interest of scientists for their low cost and the abundance of sodium resource on the earth.However,because of the disadvantages of the large radium and low charge-to-mass ratio of sodium ions,the efficient anode materials for sodium-ion batteries are highly desired and their mechanism need to be further explored.Carbon nanofibers,as a type of non-graphitic carbon,also known as hard carbon,are widely considered as an excellent candidate of anode materials for sodium-ion batteries owning to their superior electrical conductivity and mechanical properties which allows fast sodium ions insertion and extraction.Electrospinning has received considerable attention as a simple,low-cost and scalable technique for nanofiber fabrication.In this thesis,we observe the process of sodiation and desodiation in electrospun carbon nanofibers by using in-situ SEM and in-situ TEM respectively,in order to reveal the mechanism of sodium Ion storage and transport in carbon nanofiber anodes.Moreover,by making use of the residual oxygen in EM chamber,we study the phenomenon related to charging/discharging process of Na-O2 batteries.The main results of this thesis are summarized as below:A Joule heating-induced annealing process can lead to the gradual change of graphitization of carbon along the fiber axis.Sodiation experiments exhibits a much higher sodium storage capacity in amorphous regions than graphitic ones of the fiber,and mesopores inside the nanofiber can provide more space to accommodate sodium ions.Sodium ions can be inserted into and extracted from carbon nanofibers quickly after the first cycle.We discover that sodium ions are segregated from saturated carbon nanofibers to form sodium dendrites,which tend to nucleate at the mesopores along the fiber surface and can be removed by applying an inverse voltage.Besides,sodium spheres are frequently found to form at the interfaces between carbon nanofibers and sodium oxide electrolyte and metal current collector,which expand and shrink in a quasi-liquid manner.A sodium oxide shell is prone to form on the sodium sphere due to its reaction with residual oxygen in microscope chamber,thus serving as a sodium reservoir.The amount of the sodium inside the reservoir can be increased or decreased as a result of the ion transport.With the continuous sodium supply,the sphere can keep growing and break out of the sodium oxide reservoir.Whereas after the complete loss of the sodium,the sodium oxide shell decomposes and eventually disappears.In addition,sodium oxide is extremely sensitive to electron irradiation which leads to decomposition of sodium oxide into O2 and sodium(ions).This suggests that electron irradiation can somewhat promote the charging process of Na-O2 batteries.Sodium spheres encapsulated with sodium oxide shells can form and disappear along the carbon nanofiber in a reversible manner,which can indeed be utilize to fill the ubiquitous spaces in the fiber network,so as to dramatically improve the sodium capacity of carbon fiber anodes.We believe these findings will pave an inspiring new way to design the efficient sodium-ion and Na-O2 batteries.