Na7V3(P2O7)4 as a high voltage electrode material for Na-ion batteries: crystal structure and mechanism of Na+ extraction/insertion by operando X-ray diffraction

Abstract

The crystal chemistry and the electrochemical properties of Na₇V₃(P₂O₇)₄ as a high voltage cathode material upon Na⁺ extraction/insertion were investigated. The crystal structure was solved and refined for the first time from single crystal X-ray diffraction data. Possible sodium migration pathways were determined using the bond valence energy landscape (BVEL) method based on the bond valence theory. The electrochemical behavior of the material in a wide voltage range demonstrates the activity of the V³⁺/V⁴⁺, V⁴⁺/V⁵⁺, and V³⁺/V²⁺ redox couples. The reversible capacity of the Na₇V₃(P₂O₇)₄ electrode is hence increased up to 118 mA h g⁻¹vs. Na⁺/Na. Sodium extraction/insertion mechanisms are examined by operando X-ray diffraction, which provides direct information on the evolution of the material in nonequilibrium states upon electrochemical oxidation and reduction processes.