Study On PEO Based Composite Polymer Electrolytes Using Li_1.3Al_0.3Ti_1.7(PO₄)₃ As Main Component And Filler

Composite polymer electrolytes, composed of lithium ionic conductive salt (LiX) and semi-crystalline polymer, have been attracting research interests for its potential applications in rechargeable lithium batteries and other electrochemical devices. The most important polymer used as the polymeric host in composite polymer electrolyte is poly(ethylene oxide), and in short, PEO, since it can dissolve and forms complex with LiX. The ion transport in PEO-LiX systems is highly coupled with the relaxation processes of the polymer backbone and has been determined to occur predominantly in the amorphous phase of PEO. Because the crystallinity of PEO is usually high, PEO-LiX system suffers from low conductivity at room temperature. A composite polymer electrolyte with fast ion mobility and therefore high conductivities has to be of low crystallinity in its polymer matrix. In this work, PEO-based polymer electrolyte with the fast ion conductor formulated with Li1.3Al0.3Ti1.7(PO4)3 as a main ionic conductive component and as a filler in PEO-LiClO4 system were prepared by a solution-cast method. Investigations have been carried on the structure, the thermal property, the morphology and the conductivity of these composite polymer electrolytes using conventional techniques including IR, XRD, DSC, SEM and electrical impedance (EI) measurement.Based on the study of Lithium fast ion conductive systems of Li3Sc2(PO4)3, LiTi2(PO4)3 and their solid solution Li3-2x(Sc1-xTix)2(PO4)3, Li3-2x(Al1-xTix)2(PO4)3 is expected to have similar performance as a new ionic conductor. This thesis started with a systematic investigation on the preparation and characterization of Li3-2x(Al1-xTix)2(PO4)3 (X= 1.0-0.55) ceramics. Among all Li3-2x(Al1-xTix)2(PO4)3 (X = 1.0-0.55), Li1.3Al0.3Ti1.7(PO4)3 exhibits the most optimal ionic conductivity. For Li1.3Al0.3Ti1.7(PO4)3 prepared by solid reaction, the ionic conductivities of 1.792×10-6S/cm at room temperature and 9.210×10-4S/cm at 613K were obtained. Meantime, that of Li1.3Al0.3Ti1.7(PO4)3, prepared by sintering method could be as high as 7.157× 10-4S/cm at room temperature.The PEO-Li1.3Al0.3Ti1.7(PO4)3 composite polymer electrolyte with different EO/Li ratio was prepared from Li1.3 Al0.3Ti1.7(PO4)3 and PEO by the solution casting method. It was revealed that Li1.3Al0.3Ti1.7(PO4)3 is partially complexed with and partially distributed in PEO. Three phases are generally present, namely a pure crystalline PEO phase, an amorphous complexation phase and a mixture of the salt particles and amorphous PEO. The temperature dependence of ionic conductivity of PEO-Li1.3Al0.3Ti17(PO4)3 polymer electrolytes follows Arrhenius relations. The ionic conductivity of the films increases and then decreases with decreasing EO/Li, and the maximum values appear at EO/Li=16 for the whole measuring temperature range. Data for ionic transference numbers of all PEO-Li1.3Al0.3Ti1.7(PO4)3 polymer electrolyte films indicate that the charge transport was mainly attributed to ions while the polymer electrolyte was a mixed (ionic + electronic) conductor.By using ionic conductive Li1.3Al0.3Ti1.7(PO4)3 as a filler in PEO-LiClO4, PEO(LiC104)-Lii.3Alo.3Tii.7(P04)3 electrolyte films with EO/Li=8 but varying ratio of Li+ from LiC104 to Li+ from Li1.3Alo.3Ti17^04)3 were prepared by the same solution casting technique. The conductive filler effect is compared with the insulating oxide fillers effect investigated by previous researchers on the performance of the composite polymer electrolyte. It is found that PEO preferably tends to complex with LiC104 rather than Lii 3Alo.3Tii 7^04)3, and the crystallinity is considerably reduced. The temperature dependence of conductivity of all PEO(LiClO4) -Li1.3Alo.3Ti17^04)3 films follows Vogel-Tamman-Fulcher (VTF) equation, instead of Arrhenius relation. As LiijAloyTii 7(PO4)3 content is 15wt.%, the conductivity is optimal. The pseudo activation energies (Ea) and the pre-exponent constant (A) changed with the variation of ion transference number (tLi+). The ionic filler addition contributes to the impro...