Structural evolution of mixed valent (V3+/V4+) and V4+ sodium vanadium fluorophosphates as cathodes in sodium-ion batteries: comparisons, overcharging and mid-term cycling

Abstract

Sodium vanadium fluorophosphates belonging to the Na₃V₂O_2x(PO₄)₂F_3−2x family of compounds have recently shown very good electrochemical performance versus Na/Na⁺ providing high working voltages (3.6 and 4.1 V) and good specific capacity values. In this work the electrochemical behaviour and structural evolution of two compositions, Na₃V₂O_1.6(PO₄)₂F_1.4 (V^3.8+) and Na₃V₂O₂(PO₄)₂F (V⁴⁺), are detailed using time-resolved in situ synchrotron X-ray powder diffraction. For the first time in sodium-ion batteries the effects of overcharging and mid-term cycling are analyzed using this technique. Differences in the composition of both materials lead to different combinations of biphasic and single-phase reaction mechanisms while charging up to 4.3 V and overcharging up to 4.8 V. Moreover, the analysis of particle size broadening of both samples reveals the higher stress suffered by the V⁴⁺ compared to the more disordered V^3.8+ sample. The more “flexible” structure of the V^3.8+ sample allows for maximum sodium extraction when overcharging up to 4.8 V while in the case of the V⁴⁺ sample no evidence is shown of more sodium extraction between 4.3 V and 4.8 V. Furthermore, the analysis of both materials after 10 cycles shows the appearance of secondary phases due to the degradation of the material or the battery itself (e.g. electrolyte degradation). This study shows examples of the possible degradation mechanisms (and phases) while overcharging and mid-term cycling which is in turn crucial to making better electrodes, either based on these materials or generally in cathodes for sodium-ion batteries.