The study of electrochemical Li-ion (de)insertion in the lithium tantalum phosphate bronze LiTa₂PO₈ structure

Electrochemical Li⁺ (de)insertion in the lithium tantalum phosphate bronze LiTa₂PO₈ structure was performed for the first time. Using density functional theory calculations, the formation energies of various Li_1+xTa₂PO₈ (x = 0, 0.5, 1, and 1.5) configurations were obtained, and the most stable compositions were identified for each lithium content. The electron density of states is estimated for structures with minimum and maximum Li contents. Our analysis revealed that Li_1+xTa₂PO₈ (x = 0) exhibited a band gap greater than 3 eV, consistent with its known behavior as a solid electrolyte. In contrast, Li_1+xTa₂PO₈ (x = 1.5) featured a zero band gap. Then LiTa₂PO₈ was successfully prepared using the solid-state synthesis method. The measured discharge/charge capacities were equal to 67 mAh g^-1 (1.3 Li per f.u.) and 45 mAh g^-1 (0.9 Li per f.u.), which were 88% and 60% of the theoretical values. Based on the cyclic voltammetry data, both diffusion-controlled reactions and pseudocapacitive/adsorption processes were detected. High Li⁺ diffusion coefficients of 5.7 × 10^-10 cm² s^-1 (discharge) and 8.8 × 10^-10 cm² s^-1 (charge) were established. According to ex situ X-ray powder diffraction data, Li⁺ (de)insertion occurred through a solid solution and a two-phase mechanism.