Abstract
A series of Na7V4-x3+Alx(P2O7)(4)(PO4) (x = 0, 1, 2, and 4) materials were synthesized by flux crystal growth. Novel Na7V4-xAlx(P2O7)(4)(PO4) (x = 1 and 2) compositions were structurally characterized by single crystal and powder X-ray diffraction analyses using laboratory and high-resolution synchrotron X-ray sources. The investigation of the electrochemical behavior of two new mixed V/Al phases as positive electrodes in Na-based batteries was performed using charge-discharge galvanostatic tests and operando X-ray diffraction experiments. It is demonstrated that the substitution of a part of vanadium with aluminum in Na7V4(P2O7)(4)(PO4) significantly increases the gravimetric capacity of these materials: from 70.9 mA h g(-1) to 113.1 mA h g(-1) and from 48.1 mA h g(-1) to 92.7 mA h g(-1) in Na7V3Al(P2O7)(4)(PO4) and Na(7)V(2)A(2)l(P2O7)(4)(PO4), respectively thanks to the activation of the V4+/V5+ redox couple. Different degrees of reversibility of the electrochemical reactions operating on the V3+/V4+ and V4+/V5+ redox couples in Na7V3Al(P2O7)(4)(PO4) are discussed from the analysis of operando X-ray diffraction.