Li+ storage and transport in high-voltage spinel-type LiNi0.5Mn1.5O4 codoped with F- and Cu2+

The electrochemical, structural, and Li+ transport properties of dual Cu2+/F--doped LiNi0.5Mn1.5O4 (LiNi0.5Mn1.49Cu0.01O4-xFx) cathodes were probed both experimentally and theoretically and were found to strongly depend on F- content (i.e., x). Notably, some features of single Cu2+- and F--doped materials were simultaneously observed within a certain narrow range of x. LiNi0.5Mn1.49Cu0.01O3.94F0.06 (ordered P4(3)32 space group) showed the best reversible specific capacity, C-rate capability, and cyclability within the operating voltage range of 3.5-4.8 V vs. Li+/Li, while the highly fluorinated LiNi0.5Mn1.49Cu0.01O3.94F0.12 (disordered Fd3m space group) delivered a reversible discharge capacity of >200 mA h g(-1) at cut-off voltages of 4.8-2.0 V without any significant capacity fading over 50 cycles. The results of density functional theory-based simulations and experimental measurements suggested that dual doping significantly changed the electronic structure, reduced the activation energy of Li+ hopping to neighboring octahedral vacancies, and alleviated lattice distortion caused by the insertion of extra Li+ into the spinel framework, thus suppressing the irreversible transition from the spinel phase to tetragonal phases.