Understanding the cation ordering transition in high-voltage spinel LiNi_0.5Mn_1.5O₄ by doping Li instead of Ni

We determined how Li doping affects the Ni/Mn ordering in high-voltage spinel LiNi_0.5Mn_1.5O₄(LNMO) by using neutron diffraction, TEM image, electrochemical measurements, and NMR data. The doped Li occupies empty octahedral interstitials (16c site) before the ordering transition, and can move to normal octahedral sites (16d (4b) site) after the transition. This movement strongly affects the Ni/Mn ordering transition because Li at 16c sites blocks the ordering transition pathway and Li at 16d (4b) sites affects electrostatic interactions with transition metals. As a result, Li doping increases in the Ni/Mn disordering without the effect of Mn³⁺ ions even though the Li-doped LNMO undergoes order-disorder transition at 700 °C. Li doping can control the amount of Ni/Mn disordering in the spinel without the negative effect of Mn³⁺ ions on the electrochemical property.