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Title: Enhancing the conductivity of crystalline polymer electrolytes
Author: Lilley, Scott J.
Awarding Body: University of St Andrews
Current Institution: University of St Andrews
Date of Award: 2007
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The AsF6- anion, in the crystalline polymer electrolyte PEO6:LiAsF6, was replaced with the larger N(SO2CF3)2- anion. This produced an increase in the room temperature ionic conductivity of 1.5 orders of magnitude. It is believed that the enhancement is the result of the disruption of the electrostatic field around the lithium ions. The presence of the large and asymmetrical N(SO2CF3)2- ion creates a greater number of defects and thus enhances conductivity. These results demonstrate for the first time the enhancement lithium ion conductivity in a crystalline polymer electrolyte by isovalent doping. XF6- anions, in the crystalline polymer electrolyte system PEO6:LiXF6, were replaced by another anion of similar size and shape. A continuous solid solution was obtained for PEO6:(LiAsF6)1-x(LiSbF6)x. These results represent the first continuous solid solution demonstrated in the field of crystalline polymer electrolytes. They also show for the first time an enhancement of conductivity caused only by the size of the dopant anion. The enhancement is believed to originate from changes in the length of the crystal axis and changes in the potential landscape around the lithium ions. The structures of the glyme complexes monoglyme:LiAsF6, hexaglyme:LiAsF6, octaglyme:LiAsF6, undecaglyme:LiAsF6 and dodecaglyme:LiAsF6 have been solved. There structures are discussed and compared to that of PEO6:LiAsF6. The properties of these complexes together with those of diglyme:LiAsF6, triglyme:LiAsF6 and tetraglyme:LiAsF6 were investigated. Triglyme:LiAsF6 has been shown to demonstrate high ionic conductivity of 10-5.5 Scm-1 at 30oC as well as a high transport number of 0.8. These complexes demonstrate the control that crystal structure has over ionic conductivity. These complexes are neither ceramic nor polymeric. A number of the complexes show plastic crystal like solid-solid phase transitions.
Supervisor: Bruce, Peter G. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Lithium ; Solid ; Electrolytes ; Conductivity ; Electrochemistry ; Doping ; Complexes ; Crystalline