Influence of S and Ni co-doping on structure, band gap and electrochemical properties of lithium manganese oxide synthesized by soft chemical method

Ni-doped lithium manganese oxysulfides with a nominal composition of LiNixMn2−xO4−δSδ (0 ≤ x ≤ 0.5 and 0 ≤ δ ≤ 0.1) have been synthesized by an alanine-assisted, low-temperature combustion process, followed by calcination at 700 °C in air. Quantitative X-ray phase analyses show that the spinel structure of LiMn2O4 is retained for all compositions. However, analysis of the vibrational peaks observed in Fourier transformed infrared (FTIR) spectroscopy suggests that the Fd3m crystal symmetry is retained only up to x ≤ 0.4 and changes to P4332 symmetry for x = 0.5. A systematic change in microstructure is observed with increasing Ni content in presence of S. The shape of the particles changes from spherical (LiMn2O4) to icosahedron (LiNi0.2Mn1.8O4−δSδ) to octahedron (LiNi0.5Mn1.5O4−δSδ). UV–vis spectroscopy shows that the band structure of pristine LiMn2O4 is strongly influenced by hybridization among Mn 3d and O 2p orbitals near the Fermi level and the band gap (1.45 eV) gradually decreases with increasing nickel content and reaches the minimum (1.35 eV) for LiNi0.4Mn1.6O4−δSδ. Electrochemical results on 2032 coin-type cells, fabricated with the synthesized powders as the positive electrode (cathode) and Li metal as the negative electrode (anode), reveal that the substitution of S for O and Ni for Mn in LiMn2O4 enhances the structural integrity of the spinel host, which in turn increases the electrochemical cycleability. A high initial discharge capacity of 155 mAh g−1 is obtained for a LiNi0.4Mn1.6O4−δSδ/Li cell with about 87% capacity retention (135 mAh g−1) after 25 cycles at a current density of 0.2 mA cm−2. All LiNixMn2−xO4−δSδ/Li cells (x = 0.2–0.5) show excellent reversibility with nominal capacity fading (0.04–0.2 mAh per cycle) at a current density of 0.2 mA cm−2.