Lithiation mechanisms of Li₃V₂(PO₄)₃

Lithium vanadium phosphate, Li₃V₂(PO₄)₃, is widely explored in terms of the reversible extraction of three Li-ions above 3 V vs. Li. Yet, important aspects of the lithiation behavior of Li₃V₂(PO₄)₃ at low potentials remain unresolved. Reports suggest that deep lithiation leads to Li₇V₂(PO₄)₃ and even conversion to V(0) and Li₃PO₄, however, definitive structural evidence is lacking. Here, we investigate the phase evolution of Li-ion insertion in Li₃V₂(PO₄)₃ at low potential using a combination of synchrotron X-ray diffraction, scattering and spectroscopy, electrochemical analysis and void space modeling. We show that the monoclinic Li₃V₂(PO₄)₃ framework can accommodate up to two additional Li-ions, forming Li₄V₂(PO₄)₃ and Li₅V₂(PO₄)₃. Below 1.6 V the material undergoes a structural transition into a lithium vanadium phosphate phase with Li:V > 2.5, which differs from the parent Li₃V₂(PO₄)₃. At deep discharge, vanadium is reduced below an oxidation state of V(II), yet no crystalline V-metal or Li₃PO₄ is detected at any state of discharge, excluding a conventional conversion reaction at deep discharge. These findings detail the lithiation mechanism of Li₃V₂(PO₄)₃ and establish the true limits of Li storage in the anti-NASICON structure, with implications for its use as a high-capacity electrode and for the broader design of polyanionic hosts operating at low potentials.