Structural and Electrochemical Characteristics of Ca-Doped “Flower-like” Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> Motifs as High-Rate Anode Materials for Lithium-Ion Batteries

Doped motifs offer an intriguing structural pathway toward improving conductivity for battery applications. Specifically, Ca-doped, three-dimensional “flower-like” Li<sub>4–<i>x</i></sub>Ca<sub><i>x</i></sub>Ti<sub>5</sub>O<sub>12</sub> (“<i>x</i>” = 0, 0.1, 0.15, and 0.2) micrometer-scale spheres have been successfully prepared for the first time using a simple and reproducible hydrothermal reaction followed by a short calcination process. The products were experimentally characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) mapping, inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge testing. Calcium dopant ions were shown to be uniformly distributed within the LTO structure without altering the underlying “flower-like” morphology. The largest lattice expansion and the highest Ti<sup>3+</sup> ratios were noted with XRD and XPS, respectively, whereas increased charge transfer conductivity and decreased Li<sup>+</sup>-ion diffusion coefficients were displayed in EIS for the Li<sub>4–<i>x</i></sub>Ca<sub><i>x</i></sub>Ti<sub>5</sub>O<sub>12</sub> (“<i>x</i>” = 0.2) sample. The “<i>x</i>” = 0.2 sample yielded a higher rate capability, an excellent reversibility, and a superior cycling stability, delivering 151 and 143 mAh/g under discharge rates of 20C and 40C at cycles 60 and 70, respectively. In addition, a high cycling stability was demonstrated with a capacity retention of 92% after 300 cycles at a very high discharge rate of 20C. In addition, first-principles calculations based on density functional theory (DFT) were conducted with the goal of further elucidating and understanding the nature of the doping mechanism in this study. The DFT calculations not only determined the structure of the Ca-doped Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub>, which was found to be in accordance with the experimentally measured XPD pattern, but also yielded valuable insights into the doping-induced effect on both the atomic and electronic structures of Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub>.