AbstractAn 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 commercialisation of next‐generation cathodes. High‐voltage, Fe‐ and Mg‐substituted LiNi0.5Mn1.5O4 cathodes offer a low‐cost and cobalt‐free, yet energy‐dense alternative to commercial cathodes. In this work, we explore the effect of substituents on several important structure properties including Ni/Mn ordering, charge distribution and extrinsic defects. In the cation‐disordered samples studied, we observe a correlation between increased Fe/Mg substitution, Li‐site defects and Li‐rich impurity phase formation – the concentrations of which are greater for Mg‐substituted samples. We attribute this 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 were also investigated to explain the origin of Mn3+ (extrinsic defects, [Ni3+], oxygen vacancies and intrinsic off‐stoichiometry), although their effects were found to be negligible.This article is protected by copyright. All rights reserved