Structural Insights into Ni‐Stabilized Fe‐Rich High‐Voltage Spinels: LiNi_<i>x</i>Fe_{0.5−<i>x</i>}Mn_1.5O₄

Fe‐rich high‐voltage spinels are attractive positive electrode materials for next‐generation Li‐ion batteries that offer high resource efficiency and high operating voltages. However, Fe‐rich high‐voltage spinels do not provide stable cycling performance, especially when compared to the Ni‐ and Co‐rich members of the high‐voltage spinel family. To understand the failure mechanism of Fe‐rich high‐voltage spinels, the impact of Ni stabilization on the solid solutions LiNi x Fe 0.5− x Mn 1.5 O 4 ( x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) is followed. Electrochemical analysis shows that stable cycling performance can be achieved at relatively low Ni substitution ( x = 0.2). Rietveld and pair distribution function analysis show remarkable similarity in average and local structural features, supported by 4D scanning transmission electron microscopy. The cycling mechanism of LiFe 0.5 Mn 1.5 O 4 and Ni‐stabilized LiNi 0.2 Fe 0.3 Mn 1.5 O 4 is further compared via in situ powder X‐ray diffraction and in situ diffuse reflectance infrared Fourier transform spectroscopy. It is found that the presence of Ni in the electrodes leads to favorable electrolyte–electrode interactions that suppress oxidative reactions and the formation of large concentration polarizations, which is the predominant failure mechanism of LiFe 0.5 Mn 1.5 O 4 .