توصيفگر ها :
الكتروشيمي , باتري ليتيوم يون , دوپ همزمان , كاتد , كاتد NMC
چكيده انگليسي :
In this study, one of the commonly used cathode materials for lithium-ion batteries, namely NMC532 (LiNi0.5Mn0.3Co0.2O2), which is also considered the safest type for laboratory use, was investigated. In the first part of the research, the base sample was synthesized using the solid-state method. The synthesis parameters, specifically temperature and time, were selected based on previous studies, and optimization experiments were conducted accordingly. The optimization results indicated that for preparing the base NMC cathode, a sintering temperature of 950 °C and a duration of 24 h yielded a phase-pure sample free of amorphous regions. Furthermore, the splitting of the (006)/(102) and (108)/(110) doublet peaks, characteristic of long-range layered ordering, was clearly observed in the X-ray diffraction (XRD) patterns. Subsequently, Ti- and F-doped samples with 1, 3, and 5 mol% concentrations—both individually and simultaneously—were prepared and their XRD patterns analyzed. The results confirmed the absence of secondary phases in all doped specimens. The incorporation of Ti and F into the crystal structure caused slight peak shifts in the XRD patterns; thus, quantitative analyses were performed to evaluate lattice parameters. It was found that Ti doping alone led to a linear increase in unit cell volume. Fluorine doping also expanded the lattice; however, due to the solubility limit of F, the increase in cell volume plateaued at 3–5 mol%. In the co-doped samples, nonlinear variations were observed at 3 mol% Ti and 3–5 mol% F, suggesting a possible structural optimization. Microstrain analysis performed using MAUD software revealed that 3 mol% Ti doping yielded the lowest microstrain, while F doping reduced microstrain most effectively at 1 mol%. In the co-doped samples, the microstrain was generally lower than that of the pristine material. Scanning electron microscopy (SEM) observations showed that ball-milling reduced the particle size, resulting in predominantly spherical morphologies. In the milled powders, strong particle agglomeration was evident, indicating enhanced interparticle bonding compared to the raw materials. Elemental mapping of the synthesized samples confirmed a uniform elemental distribution without detectable impurities. X-ray photoelectron spectroscopy (XPS) analysis demonstrated the coexistence of Ni²⁺ and Ni³⁺ oxidation states, with minor changes in peak intensities attributed to the new bonding environments introduced by Ti and F doping. Mn and Co exhibited the highest chemical stability upon doping. The presence of Ti and F in the lattice was also confirmed. Electrochemical performance tests, including initial specific capacity, cycling stability, and rate capability, were subsequently carried out. The pristine NMC532 exhibited an initial specific capacity of 162 mAh/g, which dropped to 48 mAh/g after five cycles. While both Ti- and F-doped samples showed lower initial capacities compared to the pristine material, their capacity retention trends were similar. Among them, 3Ti and 3F displayed more favorable electrochemical performance, with initial capacities of 99 and 106 mAh/g, respectively. In the case of co-doping, the 3Ti3F sample was identified as the most promising candidate, exhibiting an initial capacity of 144 mAh/g—close to that of the pristine sample—but retaining 80 mAh/g after five cycles, a significantly improved performance relative to other samples. Electrochemical impedance spectroscopy (EIS) measurements revealed that Ti and F incorporation did not hinder lithium-ion transport within the cathode structure. From the EIS-derived data, the lithium-ion diffusion coefficients were estimated, showing that the 3Ti3F sample exhibited a diffusion coefficient approximately four times higher than that of the pristine NMC532. Consequently, 3Ti3F was identified as the optimized composition.
