ИСТИНА

NASICON-structured Na₄MnV(PO₄)₃ is taken in consideration as a cathode for sodium-ion batteries. The material is capable of irreversible de/intercalation of two sodium ions per formula unit in the potential window of 2.5-3.8 V with respect to Na/Na+ . At the same time, the structure remains stable, and the material retains its capacity after multiple charge/ discharge cycles, even at high current densities. However, with an increase of the anode potential limit, the de/intercalation mechanism changes simultaneously with a rapid loss of capacitance and an increase of potential hysteresis during continuous cycling [1]. The degradation of the electrochemical characteristics correlates with the low values of the calculated charge transfer rate constant Ks and the diffusion of sodium ions D (Fig. 1 a, b). This change can be caused by the appearance of a new amorphous phase at potentials above 3.8 V, which obstruct the intercalation of sodium ions into the structure of the material. This is confirmed by ex situ diffraction studies of electrodes based on Na₄MnV(PO₄)₃ after cycling in various charge-discharge modes (Fig. 1 c, d). A decrease in the capacity during cycling with the limit of the anode potential of 4.1 V is accompanied by an increase in the proportion of the sodium-insufficient phase in the volume of the material.