Investigation of Al³⁺ Doping Effect on LVP Cathode: Structure Evolution and Magnetic Transition

The determination of ion doping sites, doping amounts, and changes in the crystal and magnetic structures are fundamental and crucial issues that remain unclear in cation doped Li₃V₂(PO₄)₃ (LVP) cathode for lithium-ion batteries. This study employed X-ray diffraction (XRD) and Rietveld refinements, ²⁷Al/⁷Li/⁵¹V/³¹P nuclear magnetic resonance (NMR) spectroscopy, superconducting quantum interference device (SQUID) measurements, and density functional theory (DFT) calculations to investigate the doping sites and amounts of Al³⁺, the long-range and short-range crystal structures, the electronic structure, and the possible changes in magnetism in a series of Li₃V2(1_-x)Al_2x(PO₄)₃/reduced graphene oxide composites (LVA_2xP/C). The results demonstrate that the practical doping sites and amounts of Al³⁺, as well as the crystal and electronic structures, and magnetic characteristics, are affected by different ranges of Al³⁺ doping amounts. The unique signals observed in the ⁷Li 2D exchange NMR spectra, along with pairwise exchange behavior and the absence of ³¹P NMR signals, indicate changes in the magnetic behavior of LVA_2xP materials. A disruption model of orbital spin ordering was proposed based on SQUID measurement results to explain the behavior of Al³⁺-doped LVP, providing theoretical guidance for doping strategies in phosphate-based systems for lithium/sodium-ion batteries.