Structural, Electrochemical,\nand (De)lithiation Mechanism\nInvestigation of Cation-Disordered Rocksalt and Spinel Hybrid Nanomaterials\nin Lithium-Ion Batteries

Significant demand for lithium-ion batteries necessitates\nalternatives\nto Co- and Ni-based cathode materials. Cation-disordered materials\nusing earth-abundant elements are being explored as promising candidates.\nIn this paper, we demonstrate a coprecipitation synthetic approach\nthat allows direct preparation of disordered rocksalt Li<sub>2.4</sub>Fe<sub>1.0</sub>Ti<sub>1.0</sub>O<sub>4.7</sub> (r-LFTO·C) and\nspinel structured hybrid Li<sub>0.5</sub>Fe<sub>1.0</sub>Ti<sub>0.9</sub>O<sub>3.2</sub>·C (s-LFTO·C) nanoparticles with a conformal\nconductive carbon coating. High-angle annular dark-field imaging coupled\nwith electron energy loss spectroscopy mapping shows uniform Fe/Ti\ndistribution with minor compositional variation among particles. Cation\ndisorder was confirmed for both of the materials at an atomic level,\nwith a short-range order more pronounced in r-LFTO·C. Operando\nX-ray absorption spectroscopy, ex situ hard X-ray photoelectron spectroscopy,\nex situ soft X-ray absorption spectroscopy, and ex situ synchrotron\nX-ray diffraction were used to investigate (de)lithiation in the bulk\nand at the surface. Structurally, the r-LFTO·C demonstrated reversible\npartial Fe center migration between octahedral and tetrahedral sites\nduring (de)lithiation. The r-LFTO·C evidenced that the redox\nof O was coincident with iron redox during initial electrochemical\ncycling, while iron redox dominated later cycling. In contrast, s-LFTO·C\nelectrochemistry involved iron redox throughout the cycling process.\nThe findings rationalize the differences in the electrochemistry where\nr-LFTO·C shows higher initial capacity yet poorer capacity retention\nover a voltage window where O redox can be accessed, while the s-LFTO·C\nshows lower initial capacity yet improved capacity retention.