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Study of Delithiation Process Features in LixFe0.8M0.2PO4 (M = Mg, Mn, Co, Ni) by Mössbauer Spectroscopy
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. Multi-spectrum 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 charging process that is caused by doping the samples is established. Doping of
LiFePO4 leads to 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 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 determines the increase in
electronic and ionic conductivity at interfaces and rapid diffusion of lithium ions in the samples.