Electrochemical properties and evolution of the phase transformation behavior in the NASICON-type Na3+xMnxV2-x(PO4)3 (0≤x≤1) cathodes for Na-ion batteries

NASICON-structured cathode materials are considered as possible candidates for high-performance Na-ion batteries. Further increase of energy density of the Na3V2(PO4)(3) may be achieved by substitution of the V cations by other transition metals. Here, we show that a family of Na3+xMnxV2-x(PO4)(3) (0 <= x <= 1, Delta x=0.2) cathode materials demonstrates remarkable diversity of the electrochemical properties and phase transformations depending on degree of substitution and cut-off voltage. An intermediate "Na2M2(PO4)(3)" phase was found for all compounds studied by means of operando powder X-ray diffraction. When Mn content is low (x similar to 0-0.4), it coexists with Na3+xMnxV2-x(PO4)(3) or Na1+xMnxV2-x(PO4)(3). Increase in Mn content extends the length of the solid solution region corresponding to sodiated, intermediate and desodiated phases. All Mn-substituted samples are characterized by additional high-voltage plateau (-3.9 V) at charge-discharge curves. Na3+xMnxV2-x(PO4)(3) (x >= 0.4) compositions exhibit 8-10% energy density gain in comparison to Na3V2(PO4)(3) material, Na3.2Mn0.2V1.8(PO4)(3) and Na3.4Mn0.4V1.6(PO4)(3) are most preferable in terms of cycling stability.