Viologen as an electrolyte additive for extreme fast charging of lithium-ion batteries

Although lithium-ion batteries (LIBs) have found an unprecedented place among portable electronic devices owing to their attractive properties such as high energy density, single cell voltage, long shelf-life, etc., their application in electric vehicles still requires further improvements in terms of power density, better safety, and fast-charging ability (i.e., 15 min charging) for long driving range. The challenges of fast charging of LIBs have limitations such as low lithium-ion transport in the bulk and solid electrode/electrolyte interfaces, which are mainly influenced by the ionic conductivity of the electrolyte. Therefore, electrolyte engineering plays a key role in enhancing the fast-charging capability of LIBs. Here, we synthesize a novel propionic acid-based viologen that contains a 4,4′-bipyridinium unit and a terminal carboxylic acid group with positive charges that confine PF6‒ anions and accelerate the migration of lithium ions due to electrostatic repulsion, thus increasing the overall rate capability. The LiFePO4/Li cells with 0.25% of viologen added to the electrolyte show a discharge capacity of 110 mAh g‒1 at 6C with 95% of capacity retention even after 500 cycles. The added viologen not only enhances the electrochemical properties, but also significantly reduces the self-extinguishing time.