| Compared with lithium ion battery, sodium and potassium ion battery are a new type of electrochemical power and they have many advantages. For sodium ion battery, it has advantages like rich raw material resources, low cost and environment friendly. While in the case of potassium ion battery, it has good cycle performance and high power density. However, these batteries are systems which involve complex interfaces, and the charge and discharge cycles often concern electrochemical, physical, mechanical processes, so substantive exploration breakthroughs have not yet to be made in such batteries. Building a nanoscale battery structure in TEM to study the working mechanism of micro batteries in situ is a newly arisen method of battery research in recent years. In constructing a nanoscale sodium or potassium ion battery, the alkali metal is often used as positive electrode in the battery. In actual experiments, however, because alkali metal can be oxidized easily, the traditional method of scratching elementary substance from alkali metal often cannot work. In order to solve this problem, this thesis proposes to use electron beam irradiated NaCl and KCl crystals to produce Na and K metal directly, and discusses the details in the irradiation process.This thesis aims to study electron-stimulated desorption (ESD) process of NaCl and KCl using high resolution transmission electron microscopy (HRTEM). Moreover, it demonstrates the way of minimizing electron irradiation damage by encapsulating a polymerized hydrocarbon film on NaCl and KCl sample, in order to slow down the desorption rate. The main issues of this thesis are as follows:(1) Under electron beam irradiation, NaCl crystal shrinked its size gradually, and many crystalline Na colloids were found to be formed on the crystal surface or on the carbon supporting film which is away from NaCl matrix. With the increase of irradiation time, NaCl crystal ultimately disappeared and the Na colloids disappeared as well.(2) The high resolution images of NaCl desorption process show that the formed Na colloids are single crystals with body centered cubic structure, and their crystal orientations and shapes can change constantly over time. With the increase of irradiation time, the surface of Na colloids can adsorb Cl2, which is the decomposition product of NaCl, to form a NaCl layer. In the following time, this layer can also decompose by electron irradiation.(3) The high resolution images also show that under electron beam irradiation numerous rectangular structures and dislocations were formed in NaCl crystal, and the ESD process of NaCl is in accordance with "layer-by-layer" mode.(4) Under electron beam irradiation, KC1 crystal shrinked its size gradually, and many rectangular structures were found in KCl. Those rectangular structures will undergo growth and coalescent, thus restoring the plat crystal, but this process will repeat by initiating new rectangular structures on the formed flat crystal.(5) In electron beam irradiation process of KCl, we didn't find the existence of K colloids, but the the content of K element increased obviously. The results of electron diffraction show the irradiation process is accompanied by the creation of amorphous K.(6) The high resolution images of KCl desorption process show that under electron beam irradiation numerous rectangular structures and dislocations were also formed in KCl crystal.(7) A protective layer is encapsulated on NaCl and KCl crystal surface by electron beam stimulated deposition process of the native hydrocarbon contamination in TEM. The deposited thin film can encapsulate the desorption products, thus helping some of the products to recombine into sample molecules and prolonging the sample's decomposition time, to reduce electron irradiation damage. |
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