Enhancing Na⁺ Extraction Limit through High Voltage Activation of the NASICON-Type Na₄MnV(PO₄)₃ Cathode

Sodium (de)intercalation in the Na4MnV(PO4)3 NASICON-type cathode has been studied with operando synchrotron X-ray powder diffraction and galvanostatic cycling up to 3.8 and 4.0 V cutoff voltages. Symmetry reduction from rhombohedral to monoclinic was observed immediately after the start of the electrochemical desodiation, with restoring of the rhombohedral phase at 3.8 V. Cycling within the 2.5–3.8 V potential range proceeds through both solid-solution and two-phase processes. An additional voltage plateau at ∼3.9 V is observed, associated with “unlocking” of the Na1 site in the rhombohedral phase. Reverse insertion of Na+ cations proceeds via the entire solid-solution region. The experimentally observed discharge capacity increases by ≈14% after raising cutoff voltage.