Switching between solid solution and two-phase regimes in the cathode materials during lithium (de)insertion : combined PITT, in situ XRPD and electron diffraction tomography study Electrochimica acta -

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Li-rich lithium iron phosphate Li 1+δ Fe 1−δ PO 4 (Lirich LFP) prepared by the solvothermal method via the Li 3 PO 4 precursor demonstrates excellent electrochemical characteristics such as C-rate capability (140 mAh g −1 at 10 C charge for the Li 1.04 Fe 0.96 PO 4 /C material) and low voltage hysteresis between lithiation and delithiation (14 mV at C/300 rate for the same sample). Phase transformations and evolution of the Fe cations coordination environment during Li + (de)intercalation are studied in operando regime by means of synchrotron X-ray powder diffraction (SXPD) and 57 Fe Mossbauer spectroscopy (MS). The presence of a certain amount of Li + in the M2 position in the crystal structure of the as-prepared Li-rich LFPs leads to an additional component in the MS spectra corresponding to ferric ions in the M2 position with distorted second coordination sphere. Evolution of the MS spectra during charge/discharge reveals the clear relationship between the relative fraction of this component and the mechanism of Li + (de)intercalation. Extended single-phase regions with large Li + nonstoichiometry in the triphylite and heterosite phases of Li 1.04−x Fe 0.96 PO 4 observed by means of SXPD appear due to Li − Fe defects existing in the as-prepared Li-rich LFPs and acting as a "diluting" agent, which prevents two-phase transition. Increased thermodynamic stability of the intermediate Li 1+δ−x Fe 1−δ PO 4 solid solutions was also shown by DFT calculations. These features can be regarded as an additional merit of Li-rich LFPs, rendering them promising cathodes for high-power Li-ion batteries.

Peculiarities of the hydrothermal synthesis of the lithium iron phosphate cathode material are studied using in situ Calvet calorimetry. Staging and temperature intervals of the phase formation process are determined as a function of the concentration of the initial reagents. Obtained results revealed a clear correlation between observed heat absorption behavior and lattice parameters, morphology, and electrochemical performance of the obtained LiFePO 4 materials. Lowering temperature of the precursor dehydration leads to better Li/ Fe ordering, smaller particle size of LiFePO 4 samples, and the highest charge−discharge capacity measured in the Li-ion cell.