As a promising cathode material for high-energydensity Li-ion batteries, Ni-rich layered oxide cathode active materials deliver high specific capacity. However, their electrochemical performance degrades rapidly upon charge/discharge cycles probably due to electrochemical/thermochemical instabilities. While cationic doping in the transition-metal site has been regarded as an effective strategy to enhance the electrochemical performance, the true impact of cation doping is not well understood. To quantitatively assess the impact of cationic doping, in this work, the electrochemical performance and lattice oxygen stability of LiNi 0.82 Co 0.18 O 2 , isovalent Al 3+ -doped Li-Ni 0 . 8 2 Co 0 . 1 5 Al 0 . 0 3 O 2 , and high-valent Ti 4 + -doped Li-Ni 0.82 Co 0.15 Ti 0.03 O 2 were investigated. Despite significant improvements in electrochemical performance by Al 3+ and Ti 4+ doping, it was revealed that these cation dopings had no discernible effect on the lattice oxygen stability. Such information suggests that the electrochemical enhancement by Al 3+ /Ti 4+ doping is not attributed to the stabilization of lattice oxygen. This work highlights the importance of independent and quantitative experimental evaluations on kinetic electrochemical properties and thermodynamic stability of lattice oxygen to establish rational guidelines for doping strategy toward high-energy-density and reliable cathode-active materials.
Perovskite oxides, ABO3, are potential catalysts for the oxygen evolution reaction, which is important in the production of hydrogen as a sustainable energy resource. Optimizing the chemical composition of such oxides by substitution or doping with additional elements is an effective approach to improving the activity of such catalysts. Here we characterized the crystal and electronic structures of fluorine-doped La0.5Sr0.5CoO3−δ particles using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). High-resolution STEM imaging demonstrated the formation of a disordered surface phase caused by fluorine doping. In addition, spatially-resolved EELS data showed that fluorine anions were introduced into the interiors of the particles and that Co ions near the surfaces were slightly reduced by fluorine doping in conjunction with the loss of oxygen ions. Peak fitting of energy-loss near-edge structure (ELNES) data demonstrated an unexpected nanostructure in the vicinity of the surface. An EELS characterization comprising elemental mapping together with an ELNES analysis indicated that this nanostructure could not be assigned to Co-based materials but rather to the solid electrolyte BaF2. Complementary structural and electronic characterizations using STEM and EELS as demonstrated herein evidently have the potential to play an increasingly important role in elucidating the nanostructures of functional materials.
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