Increasing Tap Density of Carbon-Coated Na3V2(PO4)2F3 via Mechanical Grinding: Good or Bad Idea?

Abstract

Polyanionic positive electrode materials such as Na3V2(PO4)2F3 are renowned for their exceptional rate performance and long-term stability during cycling. However, they present low tap densities that penalize the volumetric energy density when it comes to practical applications. In this study, we successfully increased the tap density of carbon-coated Na3V2(PO4)2F3 by 40% through mechanical grinding of dense particles previously obtained via the solid-state reaction, resulting in an impressively high tap density of 1.4 g/cm3. Comprehensive structural and microstructural investigations revealed that this mechanical process reduces both particle and crystallite sizes without affecting the structure or the composition of the active material. Besides extensive electrochemical experiments, including evaluation of capacity retention upon long-term cycling and at high rates, electrochemical impedance spectroscopy as well as self-discharge tests were conducted to assess the impact of the change in microstructure on the energy storage performance. Furthermore, thermal stability assessments of electrodes in contact with electrolytes and at different states of charge were also performed to complete the study and provide a complete overview of the influence of such mechanical grinding processes commonly employed in the field of energy storage.