Influence of composition modification on Ca_0.5−xMg_xTi₂(PO₄)₃ (0.0 ≤ x ≤ 0.5) nanoparticles as electrodes for lithium batteries

Graphical abstract: - Highlights: • Cation mixing was determined in the Ca_0.5−xMg_xTi₂(PO₄)₃ biphasic series. • Nanometric Ca_0.15Mg_0.35Ti₂(PO₄)₃ delivered 138 mAh/g at C/20 in lithium cells. • Low content of Ca²⁺ increases cell volume favoring Li⁺ insertion in R-3c framework. • Diminution of R_SEI and R_CT for Ca_0.15Mg_0.35Ti₂(PO₄)₃ discharged electrodes. • Fast electrode response for x = 0.35. - Abstract: The Ca_0.5−xMg_xTi₂(PO₄)₃ series (0.0 ≤ x ≤ 0.5) was prepared by a sol–gel method. X-ray diffraction patterns showed two rhombohedral phases which coexist for intermediate compositions. Despite of the absence of a solid solution mechanism for the whole stoichiometry range, an appreciable cation mixing was observed in both phases. ³¹P MAS NMR spectroscopy revealed that low magnesium contents are incorporated to the calcium compound inducing changes in the ordering of the alkaline earth cations in M₁ sites. Derivative plots of the voltage–capacity curves revealed two reversible regions ascribed to the reduction of Ti⁴⁺ to Ti³⁺, ascribable to the subsequent insertion of lithium ions into M₁ and M₂ vacant sites. Capacity values as high as 138 mAh/g after the first discharge were monitored for nanometric Ca_0.15Mg_0.35Ti₂(PO₄)₃ at C/20. Cell cycling under successive kinetic rates revealed a good capacity retention for samples with x = 0.15 and 0.25. Impedance spectra were recorded in lithium cells discharged after different number of cycles at different C rates. The increase in charge transfer resistance was shown to be an important factor determining the electrode behavior on extended cycling