Iron-doped Sodium Vanadium Oxyflurophosphate Cathodes for Sodium-Ion Batteries—Electrochemical Characterization and In-Situ Measurements of Heat Generation

Sodium-ion batteries (NaIBs) are increasingly being envisioned for grid-scale energy storage systems because of cost advantages. However, implementation of this vision has been challenged by the low energy densities delivered by most NaIB cathodes. Toward addressing this challenge, the authors report the synthesis and characterization of a new iron-doped Na3Fe0.3V1.7O(PO4)2F2 cathode using a novel facile hydrothermal route. The synthesized material was characterized using scanning electron microscopy techniques, x-ray diffraction and Mössbauer spectroscopy. Obtained discharge capacity in half-cell configuration lies from 119 to 125 to 130 mAh/g at C/10 while tested using three different electrolyte formulations, DMC-EC-PC, DEC-EC, and EC-PC, respectively. The synthesized cathodes were also evaluated in full-cell configurations, which delivered an initial discharge capacity of 80 mAh/g with NaTi2(PO4)3MWCNT as the anode. Ionic diffusivity and interfacial charge transfer kinetics were also evaluated as a function of temperature and sodium concentration, which revealed that electrochemical rate performances in this material were limited by charge transfer kinetics. To understand the heat generation mechanism of the Na/Na3Fe0.3V1.7O(PO4)2F2 half-cell during charge and discharge processes, an electrochemical isothermal calorimetry measurement was carried out at different current rates for two different temperatures (25 and 45°C). The results showed that the amount of heat generated was strongly affected by the operating charge/discharge state, C-rate, and temperature. Overall, this work provides a new synthesis route for the development of iron-doped Na3Fe0.3V1.7O(PO4)2F2–based high-performance sodium cathode material aimed at providing a viable pathway for the development and deployment of large-scale energy storage based on sodium battery systems.