Structural and electrochemical studies of novel Na7V3Al(P2O7)4(PO4) and Na7V2Al2(P2O7)4(PO4) high-voltage cathode materials for Na-ion batteries

Abstract

A series of Na₇V_4−x³⁺Al_x(P₂O₇)₄(PO₄) (x = 0, 1, 2, and 4) materials were synthesized by flux crystal growth. Novel Na₇V_4−xAl_x(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 Na₇V₂A₂l(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.