Nano LiMn2O4 with spherical morphology synthesized by a molten salt method as cathodes for lithium ion batteries

Abstract

The compound, LiMn₂O₄ is synthesized by a one-pot molten salt method using NaCl–KCl (1 : 1) as the eutectic melt at various temperatures (T) from 700 to 850 °C and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Rietveld refinement, surface area and density methods. SEM showed that all the spinel phases LiMn₂O₄ consist of 1–5 μm sized spherical particles, each of which is composed of ∼50 nm nano-sized aggregates. TEM images show spherical particles with hollow type morphology when the synthesis T is above 800 °C. The cubic lattice parameter, 8.235 (±0.002) Å did not vary much with the T in the range 700–850 °C, which is proven by the similar Mn³⁺ and Mn⁴⁺ amount through XPS results, whereas the surface area varied from 15.6 to 10.3 m² g⁻¹. The cyclic voltammograms showed the characteristic two-step redox peaks at 3.9/4.1 and 4.1/4.2 V vs. Li for all the compounds in agreement with literature reports. Galvanostatic cycling studies were carried out in the range, 3.5 to 4.3 V vs. Li showed that the LiMn₂O₄ prepared at 800 °C has the highest discharge capacity of 124 mAh g⁻¹ at second cycle at 0.25 C-rate, and it showed a capacity retention of 96% at 1 C, 2 C and 5 C-rates at the end of 50 cycles. Long-term cycling at 2 C-rate, up to 700 cycles showed a capacity retention of 81%. Thus, LiMn₂O₄ obtained at 800 °C with uniform hollow spherical particles shows the best electrochemical properties. Complementary electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) studies were carried out and the apparent Li-ion diffusion coefficients (D_Li+) were calculated as a function of the applied voltage. The D_Li+ values from GITT range from ∼0.1 to 5 × 10⁻¹⁰ cm² s⁻¹. The values decrease with an increase in the applied voltage and show two minima in good agreement with the available literature data.