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

Abstract 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, Δx=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~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.