Defects in hydrothermally synthesized LiFePO₄ and LiFe _1-xMn_xPO₄ cathode materials

The crystal structure and defect chemistry of hydrothermally synthesized LiFe_1-xMn_xPO₄ (x = 0, 0.25, and 0.50) particles have been characterized by simultaneous neutron and X-ray Rietveld refinement as well as X-ray and neutron pair distribution function (PDF) analysis, crystallinity determination, Mössbauer spectroscopy, ion coupled plasma (ICP) studies, and scanning electron microscopy (SEM). The very detailed structural refinements show that fast hydrothermal synthesis causes partial Fe-occupancy and vacancies on the Li (M1) site, while the Fe (M2) site is always fully occupied by iron. Thus, the defect is not merely a Li/Fe antisite defect, and excessive amounts of Fe are the origin of the disorder in the structure. Neutron and X-ray total scattering with PDF analysis show that after fast hydrothermal synthesis, the crystalline, defective Li_xFe _yPO₄ coexists with amorphous Li/Fe-PO₄ structures having just short-range order. Iron excess is only seen in the crystalline part of the particles, and as the crystallinity of the samples increases with longer synthesis time, the crystalline Fe/Li ratio approaches 1. The present data thus suggest that when crystalline particles initially form, Fe is included faster in the structure from the amorphous precursor than Li, causing the defects in the structure. Only when all Li have been incorporated into the crystal structure and 100% crystallinity is achieved, fully ordered, defect free samples can be obtained. The Fe occupancy on the M1 site is therefore directly linked to the crystallinity of the sample. In LiFe _1-xMn_xPO₄ samples, the transition metal defect on the M1 site is only Fe and not Mn. Furthermore, the presence of Mn locks in the defects, and thus the Fe disorder is not suppressed with extended synthesis time.