Deciphering the role of cationic substitution towards highly stable polyanionic cathodes

For the majority of intercalation compounds as electrode materials, ion substitution offers a facile but sensible pathway toward modifying the intrinsic properties thereby boosting unexpected electrochemical performances. For polyanionic compounds, however, the underlying mechanism and site tuning of ion-doping are not well put forward owing to subtle structural variations and complex charge compensation. Herein, taking Li₃V₂(PO₄)₃ as a model, we firstly theoretically predict the lattice site preference and impurity-related defect complexes for Mg dopant under various synthesis conditions. Furthermore, the Mg-doping experimental evidence verifies that the substituted sites rely heavily on the relative abundance of lithium and the charge imbalance is mainly counterbalanced by vacant or interstitial Li, rather than hole/electron polarons. Compared to Li-site doping, V-site substituted by Mg²⁺ is preferable to form under Li-rich conditions, which causes cell volume expansion and simultaneously decreases the VO₆ octahedral distortion, realizing a highly stable structure with improved kinetics. Detailed studies on the site-dependent electronic/ionic properties elucidate the role of cation substitution in tuning electrochemical behavior, intrinsically through the energetic movement of ligand states. Specifically, selective substitution induces negatively charged anions surrounding the dopants, lowering the ligand p band center, and thus maintaining structural/interfacial stability. Our results build a clear connection between the doping site engineering and intrinsic properties for designing high-power long-term electrode materials.