Abstract
Studies of lithium iron phosphates doped with Mg, Mn, Co, and Ni were carried out at different stages of electrochemical charging using 57Fe Mössbauer spectroscopy. A multispectrum fitting method was used to analyze spectra measured at different temperatures. This made it possible to detect and characterize various local states of iron cations with different cationic environments. The feature of the charging process that is caused by doping the samples is established. Doping of LiFePO4 leads to the formation of a large interphase boundary between lithiated and delithiated regions where Fe2+ and Fe3+ cations coexist. Magnetic moments of divalent iron cations that are located near this boundary exhibit relaxation properties. Based on the obtained results, a new model of charge and discharge processes for doped samples was proposed explaining the increase of the charge/discharge rate. Within the framework of this model, regions with increased and decreased concentrations of divalent ions are formed inside the particles of a cathode material during delithiation and lithiation, respectively. The coexistence of Fe2+ and Fe3+ ions and, as a consequence, the formation of lithium defects at interphase boundaries determine the increase in electronic and ionic conductivity at interfaces and rapid diffusion of lithium ions in the samples.
Published Version
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