Abstract
A new positive electrode material, Na2.85Mn0.4V1.6(PO4)2F2.4O0.6, is synthesized via a topochemical reaction in an ionic liquid medium, starting with a tailored precursor Mn0.2(VO)0.8PO4·2H2O. Its structural and chemical characterization was conducted using a comprehensive set of techniques including X-ray diffraction, X-ray absorption, and electron paramagnetic resonance spectroscopies, as well as inductively coupled plasma optical emission spectroscopy and electron probe microanalysis. These analyses not only allowed to determine the composition and structure but also shed light on the synthesis reaction mechanism. The resulting active material exhibits promising electrochemical performance, delivering a high capacity of 108 mA h/g at a rate of C/20 with an average potential of 3.75 V vs Na+/Na. Even at a higher rate of 1C, a specific capacity of 90 mA h/g is maintained and an excellent capacity retention of 94% is demonstrated after 200 cycles at C/5. In addition, XAS analysis conducted on materials recovered at different states of charge reveals the redox activity of both manganese and vanadium centers. More generally, this work showcases the feasibility of synthesizing stable Na-deficient polyanionic phases within the NazMnxV2–x(PO4)2F3–yOy (0 ≤ x, y ≤ 2, and z ≤ 3.6) material family.