The ionic conductivity and diffusivity of NASICON-type LiTi

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.