Li‐Site Defects Induce Formation of Li‐Rich Impurity Phases: Implications for Charge Distribution and Performance of LiNi0.5−xMxMn1.5O4 Cathodes (M = Fe and Mg; x = 0.05–0.2)

An understanding of the structural properties that allow for optimal cathode performance, and their origin, is necessary for devising advanced cathode design strategies and accelerating the commercialization of next‐generation cathodes. High‐voltage, Fe‐ and Mg‐substituted LiNi0.5Mn1.5O4 cathodes offer a low‐cost, cobalt‐free, yet energy‐dense alternative to commercial cathodes. In this work, the effect of substitution on several important structure properties is explored, including Ni/Mn ordering, charge distribution, and extrinsic defects. In the cation‐disordered samples studied, a correlation is observed between increased Fe/Mg substitution, Li‐site defects, and Li‐rich impurity phase formation—the concentrations of which are greater for Mg‐substituted samples. This is attributed to the lower formation energy of MgLi defects when compared to FeLi defects. Li‐site defect‐induced impurity phases consequently alter the charge distribution of the system, resulting in increased [Mn3+] with Fe/Mg substitution. In addition to impurity phases, other charge compensators are also investigated to explain the origin of Mn3+ (extrinsic defects, [Ni3+], oxygen vacancies and intrinsic off‐stoichiometry), although their effects are found to be negligible. LiNi0.5Mn1.5O4—a high‐voltage cathode for lithium‐ion batteries—often suffers from poor cycling stability—a challenge frequently addressed through cationic substitution. Bridging experimental testing and computational modeling, this research delves into the intricacies of Fe‐ and Mg‐substituted LiNi0.5Mn1.5O4, exploring the effects on [Mn3+], [Ni3+], oxygen vacancies, defects, and impurity phases. The correlation between Li‐site defects and Li‐rich impurity phases is revealed.