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

Abstract

The electrochemical, structural, and Li⁺ transport properties of dual Cu²⁺/F⁻-doped LiNi_0.5Mn_1.5O₄ (LiNi_0.5Mn_1.49Cu_0.01O_4−xF_x) 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. LiNi_0.5Mn_1.49Cu_0.01O_3.94F_0.06 (ordered P4₃32 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 LiNi_0.5Mn_1.49Cu_0.01O_3.94F_0.12 (disordered Fdm 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.