Mastering the synergy between Na3V2(PO4)2F3 electrode and electrolyte: A must for Na-ion cells

Sodium-ion batteries are emerging as suitable energy storage devices for special applications such as high-power devices with the advantages of being cheaper and more sustainable than the Li-ion equivalents. The sodium ion cells consisting of polyanionic Na3V2(PO4)2F3 - hard carbon electrodes exhibit high power rate capabilities but limited cycle life, especially at high temperatures. To circumvent this drawback we herein conducted in-depth analyses of the origins of structural degradations occurring in Na3V2(PO4)2F3 electrodes upon long cycling. Vanadium dissolution with associated parasitic reactions is identified as one of the major reasons for cell failure. Its amount varies depending on the electrolyte, with NaTFSI-based electrolyte showing the least vanadium dissolution as the TFSI− anion decomposes without producing acidic impurities, in contrast to the NaPF6-based electrolyte. The dissolved vanadium species undergoes oxidation and reduction processes at the Na3V2(PO4)2F3 and HC electrodes, respectively, with the electrochemical signature of these processes being used as a fingerprint to identify state of health of the 18650 cells. Having found that surface reactivity is the primary cause of vanadium dissolution we provide methods to mitigate it by combining surface coating and optimized electrolyte formulation.