| Lithium ion batteries are among the category of the most competitive and widely used energy storage and conversion devices. They have experienced growing acceptance in the consumer market since they were commercialized by Sony in the early 1990s because of their favorable properties such as their high voltage, high energy density and low self-discharge. Ethylene carbonate (EC)-based electrolytes are extensively used in current commercial lithium ion batteries. However, they can only work well at above-20℃, which cannot satisfy the demand of developments of the military, space industry and polar research. With attractive low temperature performance, propylene carbonate has been shown to have the tendency to co-intercalate together with lithium cations into the layers of graphite, causing poor electrochemical properties. On the other hand, safety issue of lithium ion batteries is still a challenge due to the flamablity and unstability of liquid electrolytes. It is urged to solve the problem for the application in electric vihicles.In the thesis, methyl phenyl bis-methoxydiethoxy silane (MPBMDS) was prepared and investigated as an additive. It was found to be a bi-functional additive: preventing the exfoliation of graphite for PC-based electrolyte and acting as a flame retardant. The electrochemical properties of the electrolyte with MPBMDS were characterized by discharge/charge tests, cyclic voltammetry and electrochemical impedance spectroscopy. The mechanism of film forming was analysed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The addition of MPBMDS can effectively prevent the decomposition and the co-intercalation of PC. In addition, burning tests showed that the addition of MPBMDS to the bare PC-based electrolyte effectively reduces the flammability. This eco-friendly compound provides a new promising direction for the development of lithium ion batteries.Glass ceramic materials with NASICON structure possess attractive high temperature tolerance and high safty. They are also uniformly pore-free and easily assembled. Furthermore, the glass-ceramic materials can be manufactured to desired structure, shape and size by controlling the composition and structure. In addition, many glass-ceramic materials with NASICON structure have high ionic conductivity. Glass-ceramic materials with NASICON are expected to be widely used as solid electrolyte in batteries, electrochemical sensors and other fields. The performances of a particular glass-ceramic are determined by heat-treatment conditions of the original glass. In addition, the heat-treatment conditions play a decisive role in the size and number of micro-crystals. Crystallization conditions also could lead to a main phase transition in some systems, following with a significant change in the material properties.In this thesis, the relationship between the ionic conductivity and crystallization conditions (time and temperature) of a sodium aluminum germanium phosphate (NAGP) glass-ceramic was discussed. New sodium-ion conducting glass-ceramics composed of Na1+xAlxGe2-xP3O12 (x=0.5) crystalline conducting phase with NASICON-type structure were successfully prepared by heat-treating the parent glass at different temperatures and times. The specimens were characterized with DSC, XRD, CV, SEM and electrochemical impedance spectroscopy. The results showed that all these specimens were isomorphous with NaGe2P3O12. The impact of the heat treatment conditions on ionic conductivity was discussed by AC impedance analysis. The total ionic conductivity of the glass-ceramic material can be the highest and its possible mechanism was analysed.
|
PDF Full Text Request