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.
Andersen et al. (Thu,) studied this question.