Phosphorus Substitution in Li3VO4 Anode: Investigating Polymorphic Stability and Unconventional Redox Potential Modulation

Abstract

This study provides an in-depth investigation into the interplay between crystal polymorphs and phosphorus (P) substitution in wurtzite-type Li₃V_1–xP_xO₄ (LVPO), focusing on how crystal phase and P-substitution effects can be independently optimized to enhance electrochemical properties as anodes in lithium-ion based energy storage systems. Through precise control of the cooling rate after high-temperature synthesis, both β- and γ-phase LVPO can be reproducibly synthesized with identical P content. Powder X-ray diffraction (XRD) and in situ XRD analyses reveal that increasing P content results in a progressive stabilization of the γ-phase, demonstrating the pivotal role of P-substitution in altering the crystal structure. Electrochemical characterizations confirm that both β- and γ-LVPO exhibits smooth, single-phase (solid-solution-type) Li⁺ de/intercalation without undergoing any phase transition, a key feature that differentiates it from nonsubstituted β-Li₃VO₄. Galvanostatic intermittent titration technique (GITT) measurements show that the Li-ion diffusion coefficients follow opposing trends in β- and γ-LVPO as P content increases, providing a clear explanation for the superior rate capabilities observed in γ-LVPO. In addition, the study highlights an intriguing finding: P-substitution lowers the electrochemical redox potential, counteracting the conventional inductive effect typically reported in phosphate-based materials, thus revealing a novel mechanism by which redox behavior is sensitively influenced by local crystal environments. This work significantly advances the fundamental understanding of structure–property relationships in wurtzite-type materials, particularly in relation to how P-substitution and crystal phase transitions can optimize electrode performance. Moreover, the findings emphasize the potential of compositional and crystallographic tuning as a powerful strategy to develop high-rate anode materials with enhanced stability, improved Li⁺ diffusion, and controlled redox behavior, ultimately paving the way for the design of more efficient, stable, and high-rate lithium-ion energy systems.