Synthesis and ionic conductivity of NASICON-type lithium titanium phosphate doped with zirconium and aluminum

Today, due to the growing demand for powerful energy storage devices, lithium-ion batteries are of increasing interest. Generally, commercial samples of such energy sources are based on liquid electrolytes (solution of lithium salt in aprotic organic solvents). However, their use has a number of disadvantages: flammability and insufficient electrochemical stability. Solid electrolytes can be a promising solution to the safety problem. One of the most well-known types of solid electrolytes is lithium titanium phosphate LiTi₂(PO₄)₃ with a NASICON-type structure. Its structure is composed of TiO₆ octahedra and PO₄ tetrahedra connected by oxygen atoms, enabling for lithium-ion conductivity. To improve LiTi₂(PO₄)₃ conductivity, doping with various elements is carried out. Titanium substitution by Zr⁴⁺ leads to unit cell expansion, while Al³⁺ ion doping results in increase in Li⁺ interstitial concentration, which makes it possible to create structural defects in the lattice, in this way improving Li⁺ migration.

Thus, the aim of this work was to synthesize and study the ionic conductivity of Li_1+yTi_2-x-yZr_xAl_y(PO₄)₃ (х = 0–0.2, y = 0–0.3) compounds with the NASICON-type structure.

In this work, the morphology and ionic conductivity of the Li_1+yTi_2‑x‑yZr_xAl_y(PO₄)₃ (х = 0–0.2, y = 0–0.3) compounds prepared by both the sol-gel and solid-state methods were investigated, with the aim to study the effect of synthesis method. The Li_1+yTi_2‑x‑yZr_xAl_y(PO₄)₃ systems were studied by X-ray diffraction, scanning electron microscopy, and ³¹P NMR spectroscopy. For all materials, the dependences of ionic conductivity on temperature were evaluated by impedance spectroscopy. It is shown that the Li_1.1Ti_1.7Zr_0.1Al_0.2(PO₄)₃ composition has the highest conductivity at 25°C (7.23×10^-5 S/cm). The activation energy of Li⁺ conductivity was calculated. Its values are in the range of 34-40 kJ/mol for different materials.