The ionic conductivity and diffusivity of NASICON-type LiTi₂(PO₄)₃ and Li₃Ti₂(PO₄)₃ solid electrolytes

Materials Modelling Centre, University of Limpopo, Private Bag x1106, Sovenga, 0727, South Africa

^* Corresponding author: 201803235@myturf.ul.ac.za

Abstract

The NASICON-type solid electrolyte LiTi₂(PO₄)₃ has excellent electrochemical stability but limited ionic conductivity, largely governed by lithium-ion diffusion. One effective method to enhance this conductivity is by doping Ti⁴⁺ sites with trivalent cations like Al³⁺. This substitution introduces additional lithium ions for charge compensation, resulting in compositions such as Li₁.₃Al₀.₃Ti₁.₇(PO₄)₃. The improved ionic conductivity in such doped materials is due to the increased lithium-ion concentration. Herein, molecular dynamics simulations are employed to investigate the diffusion coefficients (D) and ionic conductivity of LiTi₂(PO₄)₃ under the isothermal–isostress (NST) ensemble and lithiated Li₃Ti₂(PO₄)₃ under the microcanonical (NVE) ensemble over the temperature range of 100 – 3100 K. In LiTi₂(PO₄)₃, lithium and oxygen Ds increase significantly above 2200 K, while titanium (Ti) and phosphorus (P) remain relatively stable. In Li₃Ti₂(PO₄)₃, oxygen D increases sharply, and lithium D peaks above 1600 K before declining after melting. The D of lithium appears to be influenced by the diffusivity of Ti and P, suggesting the structural framework significantly affects lithium mobility. At room temperature, LiTi₂(PO₄)₃ shows a low lithium D (1.14×10⁻¹¹ m²/s) and ionic conductivity (3.09×10⁻⁵ S/cm), while Li₃Ti₂(PO₄)₃ exhibits higher lithium D (2.04×10⁻¹¹ m²/s) and conductivity (2.25×10⁻⁴ S/cm), highlighting enhanced lithium transport.