Abstract
The method of self-combustion synthesis was applied to prepare double Al- and Fe-substituted LiNi0.6Mn0.2Co0.15Al0.025Fe0.025O2 (NMCAF) and non-substituted LiNi0.6Mn0.2Co0.2O2 (NMC-3:1:1) cathode materials for lithium ion batteries. The novel NMCAF structure obtained by simultaneous cationic substitution showed an improved capacity and high stability during electrochemical cycling. X-ray diffraction patterns proved that both materials have a layered α-NaFeO2 type structure with a good hexagonal ordering. It was found that NMCAF has increased a and c lattice parameters due to a structural expansion caused by Al and Fe ion substitution. Rietveld refinement analysis revealed a significant decrease of the cationic mixing after the metal substitution, suggesting a structural stabilization. Electron paramagnetic resonance (EPR) spectroscopy showed that Al and Fe substitution markedly influenced the EPR spectrum of NMC-(3:1:1). The EPR spectral lines of both materials are attributed to Mn4+ and Ni2+ present in the structure. The change in the Ni2+ line after the metal substitution suggests a redistribution of the Ni ions in the structure, which can be related to the diminished cation mixing in the NMCAF. The improved electrochemical behavior of NMCAF is closely connected to the stabilization of the layered structure and the reduction of the cation mixing after metal substitution.
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