Structural Characterization of Bimetal-Phosphate Based Solid-State Electrolytes: A PXRD, PDF and XAS Study
Date
2024
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Witwatersrand, Johannesburg
Abstract
In this work, NASICON-type lithium titanium phosphate (LiTi2(PO4)3, LTP) was synthesized following the conventional solid-state reaction methodology. Single and double-doped formulations of LTP were made, with the primary objective of improving the room-temperature ionic conductivity, for their application as potential solid-state electrolytes for all-solid-state Li ion batteries.
The primary characterization technique applied was ambient-temperature powder X-ray diffraction (PXRD) at both laboratory and synchrotron experimental conditions. The Rietveld refinement approach was used to determine the qualitative and quantitative phase compositions of each sample, revealing the rhombohedral (R-3c, space group #167) main phase, with phosphate-based secondary phases. Total scattering data, through the pair distribution function
(PDF) was applied, revealing lattice site preference during the substitution of Ti with Al, Sn and Dy at the 12c site. Further analysis through small-box modelling indicated the local structure deviation below 10 Å, from rhombohedral (R-3c) to monoclinic (P21/n, space group #14). The application of experimental X-ray absorption spectroscopy (XAS) revealed a stable 4+ oxidation state for Ti regardless of doping. However, the extended X-ray absorption fine structure (EXAFS) data showed that the replacement of Ti with Sn results in heavy disorder and subsequent changes in the PO4 tetrahedra, corroborating the findings from Raman
spectroscopy. Theoretical XAS spectra were computed using FEFF, providing insights into the origins of experimentally observed XAS features from first-principles.
Applying electrochemical impedance spectroscopy (EIS) to assess the ambient-temperature ionic conductivity, co-doped systems showed an improvement in the conductivity. The application of characterization techniques at various length scales has been demonstrated to provide insights into the mechanisms governing the performance of the solid-state electrolytes.
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy, Johannesburg 2024
Keywords
NASICON-type LTP, PXRD, Rietveld Refinement, Small-box modelling, Pair Distribution Function, X-ray absorption spectroscopy, Electrochemical impedance spectroscopy, UCTD
Citation
Nkala, Gugulethu Charmaine. (2024). Structural Characterization of Bimetal-Phosphate Based Solid-State Electrolytes: A PXRD, PDF and XAS Study [PhD thesis, University of the Witwatersrand, Johannesburg]. WireDSpace.https://hdl.handle.net/10539/41801