Lithiation mechanisms of Li3V2(PO4)3

Lithium vanadium phosphate, Li 3 V 2 (PO 4 ) 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 Li 3 V 2 (PO 4 ) 3 at low potentials remain unresolved. Reports suggest that deep lithiation leads to Li 7 V 2 (PO 4 ) 3 and even conversion to V(0) and Li 3 PO 4 , however, definitive structural evidence is lacking. Here, we investigate the phase evolution of Li-ion insertion in Li 3 V 2 (PO 4 ) 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 Li 3 V 2 (PO 4 ) 3 framework can accommodate up to two additional Li-ions, forming Li 4 V 2 (PO 4 ) 3 and Li 5 V 2 (PO 4 ) 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 Li 3 V 2 (PO 4 ) 3 . At deep discharge, vanadium is reduced below an oxidation state of V(II), yet no crystalline V-metal or Li 3 PO 4 is detected at any state of discharge, excluding a conventional conversion reaction at deep discharge. These findings detail the lithiation mechanism of Li 3 V 2 (PO 4 ) 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. • Phase evolution of Li-ion insertions in Li3V2(PO4)3 at low potentials. • Operando PXRD reveals Li4V2(PO4)3 and Li5V2(PO4)3 formation. • Deep discharge forms new monoclinic LizV2(PO4)3 (z > 5) phase. • No evidence of Li3PO4 or V metal formation at deep discharge.