Electrochemical investigation of fluorine-containing Li-salts as slurry cathode additives for tunable rheology in super high solid content NMP slurries

Abstract

Slurries with high solid contents are attractive because they can minimize usage and recycling of toxic and expensive organic solvents, but have been, so far, very challenging to realize due to their high viscosities, strong slurry gelation and poor coating results. Herein, we demonstrate the application of well-known Li electrolyte salts, namely LiTFA, LiTFSI or LiODFB, as slurry additives, which allow the achievement of an outstanding high solid content of 75.5 wt% for a NMC622-NMP slurry. These kinds of additives are chosen in order to neutralize and chemically complex the NMC622 basic surface and because of their well-known interaction within a battery system when used as electrolyte salts or additives. The investigation shows how high solid content induced slurry gelation can be tuned and controlled depending on the type of the additive and on its affinity towards the NMC622 surface. LiTFA shows the best slurry gelation controlling capabilities and LiTFSI has enhanced long-term capacity retention among the additives, rivalling the best performing reference electrode. EIS performed on fatigued cathodes after 1000 cycles shows how the contact impedance between the electrode composite and the Al current collector rises when Li-salts are used in comparison to the reference. Post-mortem SEM images show cathodic delamination for the additive-containing electrodes. Incremental capacity curves and post-mortem EDX investigations suggest Li plating on graphite anodes as a supplementary cell degradation mechanism when additives are employed.