ES22-Nkala

Author: Nkala, Gugulethu C.
1. Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag X3, Johannesburg, 2050, South Africa, 2. DSI-NRF Centre of Excellence in Strong Materials, Private Bag X3, Johannesburg, 2050, South Africa

Title: On the average, local and electronic structure of NASICON-type Li1.3Al0.25Dy0.05Ti1.7(PO4)3 (LADTP)

Abstract: Understanding structure-property correlations in materials of technological importance has been the driving force in investigating NASICON-type LiTi2(PO4)3 (LTP) systems as potential solid-state electrolytes for Li-ion batteries. Variously substituted LTP systems have been extensively investigated to increase Li+ ion conductivity in the NASICON-type structures, more specifically, the co-doped systems wherein the Ti4+ is substituted by aliovalent M3+ ions.1-3 In these studies, both strategies of enhancing Li+ ion conductivity, viz. tuning the bottleneck size by enlarging the M3+/4+O6 octahedra, and increasing the charge carrier concentration, have been employed. This is done by substituting Ti4+with a simultaneously larger and aliovalent cation at the 12c site of the rhombohedral LTP (space group R-3c). In this work, a new co-doped system, Li1.3Al0.25Dy0.05Ti1.7(PO4)3 (LADTP) is investigated. We study the average structure via synchrotron XRD and laboratory-based Raman spectroscopy, which suggest a predominantly rhombohedral (R-3c) phase. By applying small-box modelling on the pair distribution function (PDF) data, a monoclinic (P21/c) local structure up to 10 Å, showing a deviation from the average rhombohedral structure is reported for the first time. A combination of experimental and theoretical XANES supports the presence of Dy dopants in the monoclinic arrangement in the structure, in combination with a Dy3+-bearing phosphate secondary phase formed due to the ionic radii difference between Ti4+ and Dy3+. These results provide an understanding of the local and average structure that govern ionic conductivity which may facilitate further improvements on existing materials for solid-state electrolyte applications.

References 1. Nikodimos, Y., Tsai, M.C., Abrha, L.H., Weldeyohannis, H.H., Chiu, S.F., Bezabh, H.K., Shitaw, K.N., Fenta, F.W., Wu, S.H., Su, W.N. and Yang, C.C., 2020. Al–Sc dual-doped LiGe2(PO4)3–a NASICON-type solid electrolyte with improved ionic conductivity. Journal of Materials Chemistry A, 8(22), pp.11302-11313. 2. Zhang, P., Wang, H., Si, Q., Matsui, M., Takeda, Y., Yamamoto, O. and Imanishi, N., 2015. High lithium ion conductivity solid electrolyte of chromium and aluminum co-doped NASICON-type LiTi2(PO4)3. Solid State Ionics, 272, pp.101-106. 3. Kothari, D.H. and Kanchan, D.K., 2015. Study of Study of electrical properties of gallium-doped lithium titanium aluminum phosphate compounds. Ionics, 21(5), pp.1253-1259.

Other authors; Masina, Sikhumbuzo M., Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag X3, Johannesburg, 2050, South Africa Vila, Fernando D., Department of Physics, University of Washington, Seattle, WA, United States Rehr, John R., Department of Physics, University of Washington, Seattle, WA, United States Erasmus, Rudolph M., Materials Physics Research Institute, School of Physics, University of the Witwatersrand, Private Bag X3, Johannesburg, 2050, South Africa Forbes, Roy P., Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag X3, Johannesburg, 2050, South Africa Billing, Caren, Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag X3, Johannesburg, 2050, South Africa Billing, David G., 1. Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag X3, Johannesburg, 2050, South Africa, 2. DSI-NRF Centre of Excellence in Strong Materials, Private Bag X3, Johannesburg, 2050, South Africa